STATUS OF THE DOUBLE-CRESTED CORMORANT
(PHALACROCORAX AURITUS) IN NORTH AMERICA
PREPARED BY:
LINDA R. WIRES
FRANCESCA J. CUTHBERT
DALE R. TREXEL
ANUP R. JOSHI
UNIVERSITY OF MINNESOTA
DEPARTMENT OF FISHERIES AND WILDLIFE
1980 FOLWELL AVE.
ST. PAUL, MN 55108
USA
MAY 2001
PREPARED UNDER CONTRACT WITH *U.S. FISH AND WILDLIFE
SERVICE
*CONTENT MATERIAL OF THIS REPORT DOES NOT NECESSARILY
REPRESENT THE OPINIONS OF USFWS
Recommended citation: Wires, L.R., F.J. Cuthbert, D.R. Trexel and A.R. Joshi. 2001. Status of
the Double-crested Cormorant (Phalacrocorax auritus) in North America. Final Report to
USFWS.
FINAL DRAFT Executive Summary i
EXECUTIVE SUMMARY
Introduction: Since the late-1970s, numbers of Double-crested Cormorants (Phalacrocorax
auritus) (DCCO) have increased significantly in many regions of North America. A variety of
problems, both real and perceived, have been associated with these increases, including impacts
to aquaculture, sport and commercial fisheries, natural habitats, and other avian species. Concern
is especially strong over impacts to sport and commercial fishes and aquaculture. Because of
increasing public pressure on U.S. government agencies to reduce DCCO conflicts, the USFWS
is preparing an Environmental Impact Statement (EIS), and in conjunction with the U.S.
Department of Agriculture/Wildlife Services (USDA/WS) and state resource management
agencies, will develop a national management plan for the DCCO. This assessment will be used
to prepare the EIS and management plan.
Populations and trends: The DCCO breeding range in North America is divided into five
geographic areas. Since at least 1980, numbers have clearly increased in three of the breeding
areas: Canadian and U.S. interior, Northeast Atlantic Coast and Southern U.S. In these
populations, much of the growth occurred between the late 1970s – early 1990s; from the early
1990s – 2000 growth rates have slowed or appeared to stabilize in many states and provinces.
For the Pacific Coast and Alaskan breeding populations it was not possible to summarize trends
overall because recent data for birds breeding in significant portions of these regions (e.g.,
Alaska, Mexico) are not available, or have not been collected in a coordinated and timely fashion
for the populations as a whole. Along some parts of the Pacific Coast, breeding numbers
declined in the 1990s (e.g., British Columbia, species is listed as Vulnerable and is being
considered for Threatened status). In other areas significant increases occurred. Concurrently,
numbers also increased on the wintering grounds, particularly in the Mississippi River Delta
region, an area of high human-cormorant conflict over catfish resources.
Many historical records from across the continent indicate that the species was or may have been
more abundant and widespread than is currently presumed. While most of these early accounts
are largely qualitative, many report huge numbers of cormorants, suggesting that recent
population increases may represent recovery towards historical (presettlement) levels in certain
regions. In some areas where the DCCO has been documented as a recent breeder, the species is
actually re-colonizing after an absence of 50 – 300 years.
Reasons for population increases: There appear to be five major factors that led to dramatic
increases in DCCOs in North America since about 1970. These include:
1. Ban on DDT (1972) and other pesticide reduction regulation. Prior to this time (but post
WWII) widespread use of DDT occurred. Cormorants accumulated high levels of DDT
through their food supply, which interfered with reproduction. Depressed populations began
to increase after DDT was banned.
2. In 1972 the DCCO was added to the Migratory Bird Treaty Act protected bird list. Before
1972, federal legislation did not prevent killing or harassment of cormorants during their
annual cycle. Some states also provided special protection for DCCOs around this time.
FINAL DRAFT Executive Summary ii
3. Human induced changes (e.g. accidental and intentional introduction of exotics; over fishing;
changes in water quality) in aquatic communities in the breeding range.
4. Development of aquaculture (e.g. catfish farms) in the south (especially Mississippi Delta
region) that provided a new food source.
5. Creation of additional breeding and foraging habitat (e.g. reservoirs; dredge spoil islands).
Diet and native fish populations: DCCO diet is characterized by great temporal and spatial
variation. The DCCO is known to feed on > 250 species of fresh and saltwater fishes.
Cormorants are generalists and eat abundant fish in the size range 3 – 40 cm; < 15 cm is
preferred. Review of diet studies (> 40) indicates most sport and commercially valuable fish
species do not contribute substantially to DCCO diet. Though there are exceptions, most studies
conclude that sport and commercially valuable fish species are not negatively impacted by
DCCOs, and that DCCOs have minor effects on fish populations relative to human harvest and
other mortality factors. The most common claim against DCCOs is that they reduce sport or
commercial catches, but the actual relationship between cormorant predation, fish population
size and human harvest is poorly understood. This lack of information contributes to the
complexity of cormorant-fish-fishery interactions.
Rigorous quantification of cormorant predation on fish populations or on subsequent sport or
commercial catches requires more precise estimates of several key parameters, including: prey
fish population sizes; prey fish mortality sources and rates; age class distribution of fish
consumed. Additionally, a better understanding of compensatory processes within prey fish
populations is essential (e.g., predation may reduce competition so that remaining fish survive
longer or younger fish grow faster). However, no study conducted so far has obtained robust
estimates for all of these parameters. Therefore, while DCCOs may cause fish populations to
decline, none of the studies reviewed provided data rigorous enough to demonstrate that they do
so. The effect of cormorant predation can be either compensatory (if the cormorants do not eat
them, the same proportion may be removed by other factors) or additive (mortality due to
cormorant predation is not replaced by another factor). However, investigators have rarely
examined cormorant predation in the context of other mortality or limiting factors.
Because of great spatial variation in DCCO diet and unique complexities of individual aquatic
ecosystems, DCCO predation impacts need to be assessed locally. To do this biologists need a
more comprehensive understanding of local fish population dynamics and standardized methods
for assessing cormorant diet.
Diet and aquaculture facilities: Studies show DCCOs may eat large numbers of catfish locally
and temporally. However, no study has quantified the economic impact on net harvest. Only one
study has examined the issue of additive and compensatory mortality and concluded that
mortality due to DCCO predation impacts were additive under certain circumstances, but
insignificant in others.
Impacts on vegetation: Most colonial waterbirds destroy vegetation at breeding and / or
roosting sites to some extent, and cormorants cause some of the most dramatic change.
Cormorants impact vegetation through deposition of guano (excrement) that kills underlying
vegetation and eventually trees, and through nest building behavior when they strip leaves and
FINAL DRAFT Executive Summary iii
small branches. In the short term these changes are of greatest concern if they affect rare plant
communities or private property. From a long-term perspective these changes may be
insignificant on an ecosystem scale. Few studies have been conducted to characterize and
quantify vegetation change due to cormorant nesting and roosting habits.
Impacts on other bird species: DCCOs are hypothesized to have two potential effects on other
colonial waterbird species: competition for nest sites and habitat degradation. Direct interspecific
competition for nests and nest sites may occur but has not been documented through careful
study. Most impacts appear to occur indirectly through habitat degradation (e.g. defoliation, tree
die-off). While there is some evidence that DCCOs may displace other species, no studies have
clearly established DCCO impact on other birds at even a colony level scale.
Management options: Humans have attempted to manage cormorant numbers in the western
hemisphere for at least 400 years. Currently in the U.S. all lethal take requires permits from the
USFWS, except at aquaculture facilities in those states under the 1998 Federal Depredation
Order. Depredation permits can be obtained to prevent economic impacts or impacts to
endangered, threatened or species of conservation concern. Non-lethal harassment of birds
depredating or about to depredate does not require permits. To reduce cormorant impacts
primarily to fisheries, aquaculture, vegetation and other colonial waterbirds, a large number of
techniques has been developed or proposed. These techniques utilize lethal and non-lethal
measures and may be used at local, regional or population levels. The effectiveness of these
measures is difficult to assess because in many cases impacts have been poorly quantified.
Most techniques used at the local level are non-lethal. Lethal control may help reinforce local
non-lethal control techniques. However, because cormorants are highly mobile, lethal control at
the local level may be ineffective at decreasing local populations. Although economic
effectiveness cannot be assessed for individual control techniques, some appear more effective
than others; future research should focus on reducing the costs of the most promising techniques.
Many techniques have been poorly investigated; therefore conclusions about their economic and
numerical effectiveness may be premature. Because aquaculture ponds are high quality foraging
sites (high fish densities; lack of escape cover), control of cormorants on the breeding grounds is
unlikely to eliminate the need to practice local control. To make aquaculture ponds less desirable
foraging sites, some form of control at the local level (e.g. exclosures, harassment) will likely
still be needed. Previous efforts indicate that population control in general must be large scale
and will require sizable human and capital inputs to be effective. Additionally, potential density
dependent effects that compensate for control related mortality are poorly understood.
Addressing these and numerous other uncertainties will enhance the development of a
scientifically based, large-scale population control effort.
Finally, no control is a management option that is economically justified if the costs of control
are greater than the losses associated with cormorant impacts.
Population Models: Models have identified data gaps critical for understanding population
dynamics and predicting control effectiveness; modeling is potentially a very strong tool for
gaining insights into cormorant management. Prediction of future DCCO population trends and
analysis of control methods is hampered by lack of age-specific data for this species. More effort
FINAL DRAFT Executive Summary iv
needs to be put into obtaining data needed to strengthen model predictions, and increased effort
should focus on predicting management outcomes and follow progress. Until better data are
available, however, such modeling efforts should include rigorous sensitivity analyses to
investigate uncertainties in parameters used and assumptions made in the model.
Current research and monitoring efforts: Of 33 U.S. states and nine Canadian provinces to
which surveys were sent, nine reported research in progress and 19 have monitoring programs.
Research addresses: cormorant diet, bioenergetics, impacts to aquaculture, sport and commercial
fisheries, foraging range and foraging behavior. Additional studies are attempting to determine
effectiveness of harassment at day and night roosts, effectiveness of barriers at aquaculture
ponds, and nutrient enrichment in aquatic and terrestrial habitats. A satellite telemetry study will
determine migration patterns, breeding locations and winter movements of cormorants at catfish
farms. All monitoring efforts are used to determine population distribution and trends.
Future research priorities: The assessment identified many research needs. Highest priority
studies on DCCOs fall within the following broad topics: (1) demography, (2) impacts on
fisheries and aquaculture, (3) management techniques, (4) impacts on flora and fauna and (5)
distribution.
FINAL DRAFT List of Figures v
LIST OF FIGURES
Figure 1. Distribution of Double-crested Cormorant (Phalacrocorax auritus) breeding colonies in
North America 1970 – 2000. .............................................................................................15
Figure 2. DCCO Christmas Bird Count sites (1984 – 1998) where ³ 100 birds were counted at
least once. .........................................................................................................................16
Figure 3. December estimates of DCCOs in Alaska based on CBCs, 1984 – 1998.....................18
Figure 4. Distribution and size of active colonies in Alaska at time of last surveys (1970 – 1999).
.........................................................................................................................................19
Figure 5. December estimates of DCCOs on the Pacific Coast, Zone 2*, based on CBCs, 1984 –
1998.................................................................................................................................23
Figure 6. Distribution of Pacific Coast DCCO breeding population (Zone 2), 1970 – 2000. ......25
Figure 7. Distribution and size of active colonies on north Pacific Coast at time of last surveys
(1990s).............................................................................................................................26
Figure 8. Distribution and size of active colonies on south Pacific Coast at time of last surveys
(CA = 1989 – 1998; MX = 1973 – 1992)...........................................................................27
Figure 9. Distribution and size of active colonies in the Pacific Coast southwestern states (1993 –
2000)................................................................................................................................28
Figure 10. December estimates of DCCOs in Arizona based on CBCs, 1984 – 1998. ................30
Figure 11. Changes in number of pairs at Great Chain, Five Finger and Mandarti Islands, Gulf
Islands, B.C., 1987 – 2000.................................................................................................33
Figure 12. December estimates of DCCOs in British Columbia based on CBCs, 1984 – 1998...34
Figure 13. December estimates of DCCOs in California based on CBCs, 1984 – 1998..............38
Figure 14. December estimates of DCCOs at the Salton Sea based on CBCs, 1984 – 1999........38
Figure 15. Changes in number of breeding pairs at Anaho Island, Pyramid Lake, Nevada, 1986 –
1998.................................................................................................................................42
Figure 16. December estimates of DCCOs in New Mexico based on CBCs, 1984 – 1998. ........44
Figure 17. Changes in number of breeding pairs at East Sand and Rice Islands, Columbia River
Estuary, Oregon, 1991 – 2000. ..........................................................................................47
Figure 18. Changes in numbers of breeding pairs at Malheur and Upper Klamath Lakes, 1986 –
1999.................................................................................................................................47
Figure 19. December estimates of DCCOs in Oregon based on CBCs, 1984 – 1998..................48
Figure 20. Changes in breeding numbers at colonies located along the Washington outer coast,
1979-1999.........................................................................................................................50
Figure 21. Changes in breeding numbers at colonies located in theWashington inner coastal
waters, 1983-2000.............................................................................................................51
Figure 22. Changes in breeding numbers in interior Washington, N. Potholes Reservoir, 1978 –
1997.................................................................................................................................51
Figure 23. December estimates of DCCOs in Washington based on CBCs, 1984 – 1998...........52
Figure 24. Changes in numbers of breeding pairs on the Great Lakes, 1970 – 1997...................56
Figure 25. Distribution of Interior U.S. and Canadian DCCO breeding population (Zone 3), 1979
– 2000..............................................................................................................................58
Figure 26. Distribution and size of active colonies in Saskatchewan during last complete survey
(1991). ..............................................................................................................................59
Figure 27. Distribution and size of active colonies on Lake Winnipegosis during last complete
survey (1999). ...................................................................................................................60
FINAL DRAFT List of Figures vi
Figure 28. Distribution and size of active colonies in the Interior west-central states during last
complete surveys (1986 – 1999). .......................................................................................61
Figure 29. Distribution and sizes of active colonies in Interior mid-central states at time of last
complete surveys...............................................................................................................62
Figure 30. Distribution of active colonies (size data not available) in South Dakota and
Minnesota (MN = 1981 – 1995; SD = 1988 – 1992). .........................................................63
Figure 31. Distribution and size of active colonies in Wisconsin during last complete survey
(1997). ..............................................................................................................................64
Figure 32. Distribution and size of active colonies in Michigan during last complete survey
(1997). ..............................................................................................................................65
Figure 33. Distribution and size of active colonies in Ontario during last complete survey (1997).
.........................................................................................................................................66
Figure 34. Detail of Lake Huron showing size and distribution of active colonies (1997). .........67
Figure 35. Distribution and size of active colonies in Ohio, New York, and Vermont................68
Figure 36. Changes in numbers of breeding pairs in Illinois, 1986 – 1997. ................................73
Figure 37. Changes in breeding numbers on Lake Winnipegosis, 1979 – 1999. .........................79
Figure 38. Changes in numbers of breeding pairs in Michigan, 1977 – 1997. ............................81
Figure 39. Changes in number of breeding pairs at Goose Lake, Crescent Lake NWR, Nebraska,
1973 – 1996. .....................................................................................................................86
Figure 40. Changes in number of breeding pairs at Marsh Lakes, Valentine NWR, Nebraska,
1977 – 2000. .....................................................................................................................86
Figure 41. Changes in number of breeding pairs on Little Galloo Island, 1977 – 1998...............88
Figure 42. Changes in numbers of breeding pairs in Ohio, 1992 – 1998. ...................................92
Figure 43. Changes in breeding numbers in Ontario, 1973 – 1997.............................................94
Figure 44. Changes in breeding numbers in Saskatchewan, 1968 – 1991...................................95
Figure 45. Changes in breeding numbers at Bear River Refuge, Utah, 1971 – 1996...................98
Figure 46. Changes in numbers of breeding pairs in Vermont, 1982 – 1999. ...........................100
Figure 47. Changes in breeding numbers in Wisconsin, 1973 – 1997. .....................................102
Figure 48. December estimates of DCCOs in the Southeastern U.S., Zone 4,* based on CBCs,
1984 – 1997. ...................................................................................................................107
Figure 49. Distribution of South and Southeastern U.S. DCCO breeding population (Zone 4)
1980 – 1999. ...................................................................................................................109
Figure 50. Distribution and size of active colonies in South Carolina during last comnplete
survey (1996). .................................................................................................................110
Figure 51. Comparison of Christmas Bird Count sites (with ³ 100 birds) with roost sites located
by USDA/WS in Arkansas, Mississippi and Alabama. ....................................................111
Figure 52. December estimates of DCCOs in Alabama based on CBCs, 1984 – 1997. ............113
Figure 53. December estimates of DCCOs in Arkansas based on CBCs, 1984 – 1997. ............115
Figure 54. December estimates of DCCOs in Florida based on CBCs, 1984 – 1997 ................118
Figure 55. December estimates of DCCOs in Georgia based on CBCs, 1984 – 1997. ..............119
Figure 56. December estimates of DCCOs in Louisiana based on CBCs, 1984 – 1997. ...........123
Figure 57. December estimates of DCCOs in Mississippi based on CBCs, 1984 – 1997..........125
Figure 58. Double-crested Cormorant from Brickell (1737), possibly the first published
representation of this species. ..........................................................................................127
Figure 59. December estimates of DCCOs in North Carolina based on CBCs, 1984 – 1997. ...127
Figure 60. December estimates of DCCOs in Oklahoma based on CBCs, 1984 – 1997. ..........129
FINAL DRAFT List of Figures vii
Figure 61. Changes in number of breeding pairs in South Carolina, 1989 – 1996. ...................131
Figure 62. December estimates of DCCOs in South Carolina based on CBCs, 1984 – 1997. ...132
Figure 63. Changes in number of breeding pairs in Tennessee, 1992 – 1999. ..........................134
Figure 64. December estimates of DCCOs in Tennessee based on CBCs, 1984 – 1997. ..........134
Figure 65. December estimates of DCCOs in Texas based on CBCs, 1984 – 1997. .................137
Figure 66. Changes in numbers of breeding pairs in New England, 1977 – 1994/95. ...............141
Figure 67. December estimates of DCCOs in the Northeast Atlantic, Zone 5*, based on CBCs,
1984 – 1998. ...................................................................................................................142
Figure 68. Distribution of Northeast Atlantic Coast DCCO breeding population (Zone 5), 1980 –
1999...............................................................................................................................145
Figure 69. Distribution and size of active colonies in the mid-Atlantic states at time of last
complete surveys.............................................................................................................146
Figure 70. Distribution and size of active colonies in New England at time of last surveys......147
Figure 71. Distribution and size active colonies in Maine during last complete survey (1994 –
1995)..............................................................................................................................148
Figure 72. Distribution and size of active colonies in New Brunswick, Novia Scotia and Prince
Edward Island during last complete surveys. ...................................................................149
Figure 73. Distribution of active colonies in Quebec at time of last surveys (1970 – 1998)......150
Figure 74. Distribution and size of active colonies in Quebec at time of last surveys (1970 –
1998)..............................................................................................................................151
Figure 75. Detail of the St. Lawrence River Estuary and Gaspe Peninsula showing distribution
and size of active colonies at time of last surveys (1970 – 1998)......................................152
Figure 76. Distribution and size of active colonies along the coast of Newfoundland and on
offshore islands during last complete survey (1996). .......................................................153
Figure 77. Changes in numbers of breeding pairs in Connecticut, 1986 – 1998. ......................155
Figure 78. Changes in numbers of breeding pairs in Maine, 1977 – 1994/95. ..........................158
Figure 79. Changes in number of breeding pairs in Maryland, 1990 – 1999.............................159
Figure 80. Changes in numbers of breeding pairs in Massachusetts, 1977 – 1994/5.................161
Figure 81. December estimates of DCCOs in New Jersey based on CBCs, 1984 – 1998..........166
Figure 82. Changes in breeding numbers in New York (coastal) 1985 – 1998. ........................168
Figure 83. Changes in breeding numbers in Nova Scotia, 1972 – 1992....................................170
Figure 84. Changes in numbers of breeding pairs on Prince Edward Island, 1976 – 1999. .......172
Figure 85. Changes in breeding numbers in Québec, 1970 – 1990...........................................175
Figure 86. Changes in breeding numbers in Rhode Island, 1981 – 1998. .................................177
Figure 87. Annual totals of DCCOs in Virginia based on CBCs, 1984 – 1998. ........................179
Figure 88 ...............................................................................................................................241
Figure 89. The relationship between annual percent increase and r..........................................286
Figure 90. Numbers of Apparently Occupied Nests (AON) on Pigeon Island, Ontario with
population growth estimates. ...........................................................................................286
Figure 91. Annual percent change in number of apparently occupied nests (AON) in Pigeon
Island, Lake Ontario........................................................................................................287
Figure 92. Nesting population change on Gull Island, Lake Superior and logistic model based on
1978 – 1985 nest counts. .................................................................................................294
FINAL DRAFT List of Tables ix
LIST OF TABLES
Table 1. Zone 2, Pacific Coast. Summary of Population data and trends. (+ = increasing; - =
decreasing; 0 = no change). ...............................................................................................24
Table 2. Numbers of chicks fledged at DCCO colonies along the east coast of Vancouver Island
(from Moul 2000)..............................................................................................................33
Table 3. Summary of most recent census data for California breeding regions...........................37
Table 4. Zone 3, Canadian and U.S. Interior. Summary of Population Data and Trends. (+ =
increasing; - = decreasing; 0 = no change).........................................................................57
Table 5. Zone 4, Southern U.S. Summary of Population Data and Trends (+ = increasing; - =
decreasing; 0 = no change). .............................................................................................108
Table 6. Zone 5, Northeast Atlantic Coast. Summary of Population Data and Trends (+ =
increasing; - = decreasing; 0 = no change).......................................................................143
Table 7. Pacific Coast: Occurrence of Salmonids in the diet of Double–crested Cormorants....208
Table 8. Pacific Coast: Occurrence of littoral or estuarine fish in the diet of Double–crested
Cormorants. ....................................................................................................................209
Table 9. Pacific Coast: Occurrence of littoral-benthic fish in the diet of Double–crested
Cormorants. ....................................................................................................................211
Table 10. Pacific Coast: Occurrence of pelagic fish in the diet of Double–crested Cormorants.
.......................................................................................................................................212
Table 11. Great Lakes: Occurrence of open-fresh water fish in the diet of Double–crested
Cormorants. ....................................................................................................................213
Table 12. Great Lakes: Occurrence of littoral-freshwater fish in the diet of Double–crested
Cormorants. ....................................................................................................................215
Table 13. Great Lakes: Occurrence of bottom dwelling-fresh water fish in the diet of Double–
crested Cormorants..........................................................................................................219
Table 14. Great Lakes: Occurrence of Salmonids in the diet of Double–crested Cormorants. ..221
Table 15. Great Lakes: Locally important fish species in the diet of Double–crested Cormorants.
.......................................................................................................................................222
Table 16. Southern U.S.: Occurrence of Shad in the diet of Double–crested Cormorants.........223
Table 17. Southern U.S.: Occurrence of Catfish in the diet of Double–crested Cormorants. ....224
Table 18. Southern U.S.: Occurrence of Centrarchids in the diet of Double–crested Cormorants.
.......................................................................................................................................225
Table 19. Southern U.S.: Locally important fish species in the diet of Double–crested
Cormorants. ....................................................................................................................226
Table 20. Atlantic Region: Occurrence of Marine bottom fish in the diet of Double–crested
Cormorants. ....................................................................................................................226
Table 21. Atlantic Region: Occurrence of Estuarine / Diadromous fishes in the diet of Double–
crested Cormorants..........................................................................................................229
Table 22. Atlantic Region: Occurrence of Freshwater fishes in the diet of Double–crested
Cormorants. ....................................................................................................................232
Table 23. Atlantic Region: Occurrence of Salmonids in the diet of Double–crested Cormorants.
.......................................................................................................................................234
Table 24. Atlantic Region: Occurrence of Pelagic fishes in the diet of Double–crested
Cormorants. ....................................................................................................................235
FINAL DRAFT List of Tables x
Table 25. Atlantic Region: Occurrence of Crustaceans in the diet of Double–crested Cormorants.
.......................................................................................................................................236
Table 26. Sources of information on DCCO control at aquaculture facilities. ..........................243
Table 27. Percent of winter band recoveries (1923-1988) that occurred in the Lower Mississippi
Valley for birds originally banded during the nesting season in various geographic regions
(Dolbeer 1991)................................................................................................................260
Table 28. States and provinces practicing Double-crested Cormorant control. .........................282
Table 29. Impacts in Zone 3 (U.S. and Canadian Interior).......................................................333
Table 30. Impacts in Zone 4 (Southern U.S.)...........................................................................333
Table 31. Impacts in Zone 5 (North Atlantic) ..........................................................................334
Table 32. Summary of fisheries survey data: states / provinces reporting declines linked to
DCCOs. ..........................................................................................................................335
Table 33. Summary of Fisheries Survey Data: Fish species reported to be in decline and factors
thought responsible. ........................................................................................................336
Table 34. States and provinces engaged in current DCCO or DCCO related research or
monitoring programs, and or with identified research priorities. ......................................337
FINAL DRAFT Acknowledgements xi
ACKNOWLEDGEMENTS
Of the hundreds of individuals who contributed to this review, five provided significant
intellectual insight, guidance and friendship: (1) Jeremy Hatch carefully reviewed multiple drafts
of this and related manuscripts, and never failed to provide copious (constructive) criticisms. His
suggestions on organization of the population data were exceptionally helpful and shaped the
entire document. (2) D.V. (Chip) Weseloh, through review of the manuscript and numerous
conversations, provided important ideas on the biology and management of cormorants. His
consistent support, cooperation and encouragement were invaluable. (3) David Carss contributed
in ways too numerous to mention. His publications, review of this assessment, and his
enthusiasm for discussing transatlantic cormorant issues revolutionized our thoughts on
cormorant diet assessment and impacts to fisheries. (4) Thomas Bregnballe provided and helped
us obtain the extensive literature on the Great Cormorant in Europe. He contributed information
that helped broaden our understanding of the complexity of population-level management, and
his work and insights were inspirational. (5) Thomas Keller provided stimulating ideas about
trans-continental cormorant conflicts, particularly on diet, human perceptions and management
issues.
Over fifty people reviewed earlier drafts of this assessment. We are grateful for all of their
comments and suggestions. We especially thank Connie Adams, Robert Ross and Jim Glahn for
providing careful, detailed reviews that challenged our ideas and interpretations, and made us
consider complex issues from multiple perspectives.
Other persons who contributed to our thoughts on cormorants, development of the status
assessment, and problems with piscivorous birds in general include Bill Montevecchi, Bill
Koonz, Walt Lysack and John Trapp. Also, we are grateful to Steve Lewis and Shauna Hanisch
for their support, encouragement and patience; Bruce Vondracek helped organize ecological
groupings for fish species and answered many questions related to fish population dynamics.
Finally, we cannot thank enough the numerous persons identified in Appendix V who
contributed information on biology, distribution, current numbers and recent changes of the
Double-crested Cormorant in North America. Their dedication enabled us to compile an accurate
and detailed status assessment of this species.
The U.S. Fish and Wildlife Service funded preparation of this assessment.
FINAL DRAFT Table of Contents xiii
TABLE OF CONTENTS
EXECUTIVE SUMMARY..........................................................................................................i
LIST OF FIGURES ....................................................................................................................v
LIST OF TABLES..................................................................................................................... ix
ACKNOWLEDGEMENTS ....................................................................................................... xi
TABLE OF CONTENTS......................................................................................................... xiii
INTRODUCTION ......................................................................................................................1
Background Information .........................................................................................................1
The Status Assessment ............................................................................................................2
PART I: NATURAL HISTORY OF THE DOUBLE-CRESTED CORMORANT, Phalacrocorax
auritus .......................................................................................................................................3
GENERAL INFORMATION..................................................................................................3
Taxonomy..........................................................................................................................3
Physical Description............................................................................................................3
Range.................................................................................................................................4
Breeding Range. ..............................................................................................................4
Winter Range. .................................................................................................................4
Habitat ...............................................................................................................................5
Breeding Season Habitat Requirements. ..........................................................................5
Winter Habitat Requirements. .........................................................................................5
BIOLOGY.............................................................................................................................5
Migration ............................................................................................................................5
Reproduction......................................................................................................................6
Foraging Ecology................................................................................................................7
Diet ....................................................................................................................................8
Longevity, Survival and Mortality.......................................................................................8
PART II. POPULATION DATA AND TRENDS.......................................................................9
INTRODUCTION ..................................................................................................................9
Breeding Birds ....................................................................................................................9
Biologically Relevant Units and Organization of Data.....................................................9
Colony Locations ..........................................................................................................10
Trends ...........................................................................................................................11
Wintering Birds.................................................................................................................11
Locations and Winter Distribution.................................................................................11
Use of Christmas Bird Count Data.................................................................................11
Wintering Numbers and Trends .....................................................................................12
Migrant Birds....................................................................................................................12
Migration / Stop-over Sites............................................................................................12
Trends ...........................................................................................................................12
Non-breeding Birds and Total Population Size ..................................................................13
ZONE 1: SUMMARY OF POPULATION DATA FOR ALASKA.......................................17
Introduction......................................................................................................................17
Alaska..............................................................................................................................17
ZONE 2: SUMMARY OF POPULATION DATA FOR THE PACIFIC COAST..................21
FINAL DRAFT Table of Contents xiv
Introduction......................................................................................................................21
Breeding Information ....................................................................................................21
Winter and Migrant Birds..............................................................................................22
Historical Information ...................................................................................................22
Arizona .............................................................................................................................29
British Columbia ...............................................................................................................30
California ..........................................................................................................................34
Mexico.............................................................................................................................39
Nevada.............................................................................................................................41
New Mexico......................................................................................................................42
Oregon.............................................................................................................................44
Washington......................................................................................................................48
ZONE 3: SUMMARY OF POPULATION DATA FOR CANADIAN AND U.S. INTERIOR
POPULATION .....................................................................................................................53
Introduction......................................................................................................................53
Breeding Information ....................................................................................................53
Winter and Migrant Birds..............................................................................................54
Historical Information ...................................................................................................54
Alberta.............................................................................................................................69
Colorado ...........................................................................................................................69
Idaho................................................................................................................................70
Illinois..............................................................................................................................72
Indiana ..............................................................................................................................73
Iowa.................................................................................................................................74
Kansas ..............................................................................................................................76
Manitoba..........................................................................................................................77
Michigan..........................................................................................................................79
Minnesota .........................................................................................................................81
Missouri...........................................................................................................................83
Montana...........................................................................................................................84
Nebraska ...........................................................................................................................85
New York (Inland) ............................................................................................................87
North Dakota.....................................................................................................................89
Ohio .................................................................................................................................90
Ontario ..............................................................................................................................92
Saskatchewan...................................................................................................................94
South Dakota ....................................................................................................................96
Utah .................................................................................................................................97
Vermont...........................................................................................................................99
Wisconsin .......................................................................................................................100
Wyoming ........................................................................................................................102
ZONE 4: SUMMARY OF POPULATION DATA FOR SOUTHEASTERN U.S. ..............105
Introduction....................................................................................................................105
Zone Boundaries. ........................................................................................................105
Breeding Information ..................................................................................................105
Winter and Migrant Birds............................................................................................105
FINAL DRAFT Table of Contents xv
Historical Information .................................................................................................106
Alabama.........................................................................................................................112
Arkansas .........................................................................................................................113
Florida ............................................................................................................................116
Georgia ...........................................................................................................................118
Kentucky........................................................................................................................120
Louisiana ........................................................................................................................121
Mississippi.....................................................................................................................123
North Carolina ................................................................................................................125
Oklahoma.......................................................................................................................128
Puerto Rico / Virgin Islands ............................................................................................130
South Carolina ................................................................................................................130
Tennessee.......................................................................................................................132
Texas ..............................................................................................................................135
ZONE 5: SUMMARY OF POPULATION DATA FOR THE NORTHEAST ATLANTIC
COAST..............................................................................................................................139
Introduction....................................................................................................................139
Breeding Information ..................................................................................................139
Winter and Migrant Birds............................................................................................140
Historical Information .................................................................................................140
Summary of Population Data and Trends.....................................................................154
Historical Information .................................................................................................154
Land Ownership ..........................................................................................................154
Productivity.................................................................................................................154
Delaware........................................................................................................................155
Maine.............................................................................................................................156
Maryland........................................................................................................................158
Massachusetts .................................................................................................................160
New Brunswick...............................................................................................................162
Newfoundland.................................................................................................................163
New Hampshire ..............................................................................................................164
New Jersey......................................................................................................................165
New York (Coastal) ........................................................................................................166
Nova Scotia.....................................................................................................................168
Pennsylvania ...................................................................................................................170
Prince Edward Island ......................................................................................................171
Québec...........................................................................................................................173
Rhode Island ...................................................................................................................176
Virginia..........................................................................................................................177
West Virginia ..................................................................................................................179
PART III: DIET......................................................................................................................181
Introduction ........................................................................................................................181
Diet Studies Reviewed ....................................................................................................181
Diet Assessment Methods: Advantages and Shortcomings ..............................................181
Pellets .........................................................................................................................182
Regurgitations .............................................................................................................183
FINAL DRAFT Table of Contents xvi
Stomach Contents........................................................................................................184
Recovered Tags From Stocked Fish.............................................................................185
Conclusions: Diet Assessment Methods ......................................................................185
Measurements Used to Report Diet .................................................................................185
Results of Diet Review........................................................................................................186
Regional Diet Descriptions..............................................................................................186
Diet on the Pacific Coast .............................................................................................186
Diet in the Great Lakes................................................................................................187
Diet in the Southeastern U.S........................................................................................187
Diet in the Atlantic Region ..........................................................................................188
Conclusions: Variation in the Diet ...............................................................................188
Fish species declines associated with DCCO predation (perceived or real) ......................189
Studies assessing impacts to sport and / or commercial fish populations ..........................190
Great Lakes .................................................................................................................190
Southeastern U.S.........................................................................................................197
Conclusions: Impacts to Sport and / or Commercial Fisheries......................................199
Studies assessing impacts to aquaculture .........................................................................200
Surveys .......................................................................................................................201
Diet Studies and Observations at Ponds.......................................................................202
Bioenergetics Modeling...............................................................................................205
Conclusions: Impacts to Aquaculture...........................................................................207
Studies on food preferences.............................................................................................207
PART IV. IMPACTS TO VEGETATION AND COLONIAL WATERBIRDS ......................237
Impacts to Vegetation .........................................................................................................237
Impacts to avian species ......................................................................................................238
Newcastle Disease ..............................................................................................................239
PART V. MANAGEMENT OPTIONS...................................................................................241
Introduction ........................................................................................................................241
Managing Impacts to Aquaculture.......................................................................................244
1) Exclusion Devices: Physical and Functional Barriers ..................................................244
Netting ........................................................................................................................244
Wires..........................................................................................................................245
Floating Ropes ............................................................................................................246
Other Flight Inhibitors.................................................................................................247
Underwater Exclusion Devices....................................................................................247
2) Non-lethal Harassment at Aquaculture Facilities .........................................................247
Human Harassment .....................................................................................................247
Simulated Human Harassment.....................................................................................249
Other Harassment ........................................................................................................250
3) Lethal Control at Aquaculture Facilities ......................................................................251
Reinforcing Non-lethal Harassment Techniques ..........................................................251
Reducing Local Populations ........................................................................................252
Accidental Killing of Other Species.............................................................................253
4) Non-lethal Harassment at Night Roosts.......................................................................254
5) Provision of Alternative Prey and Foraging Sites ........................................................255
Provision of Alternative Prey.......................................................................................255
FINAL DRAFT Table of Contents xvii
Alternative Foraging Sites ...........................................................................................256
6) Alteration of Aquaculture Practices.............................................................................256
Decrease Fish Densities...............................................................................................257
Pond Dyes...................................................................................................................257
Water Turbulence........................................................................................................257
Aquaculture Facility Location .....................................................................................258
Improved Facility Design ............................................................................................258
Alter Stocking Practices ..............................................................................................258
Passive Harassment .....................................................................................................259
Alteration of Fish Behavior .........................................................................................259
Chemical Avian Repellants .........................................................................................259
Conditioned Taste Aversion ........................................................................................259
Manage Other Avian Species.......................................................................................259
7) Population Control......................................................................................................259
8) No Control..................................................................................................................262
9) Conclusions: Managing Impacts to Aquaculture..........................................................262
Managing Impacts to Vegetation and Habitat ......................................................................263
Techniques.....................................................................................................................264
Harassment of Adults ..................................................................................................264
Nest / Tree Removal....................................................................................................265
Egg and Nestling Destruction ......................................................................................266
Egg Oiling / Spraying..................................................................................................268
Avian Contraceptives ..................................................................................................270
Killing Adults..............................................................................................................270
No Control ..................................................................................................................272
Case Studies: Multiple Techniques..................................................................................272
Lake Winnipegosis, Manitoba .....................................................................................273
St. Lawrence Estuary, Québec .....................................................................................274
Lake Champlain, Vermont (USFWS 1999a)................................................................275
Lake Ontario, New York (USFWS 1999b) ..................................................................277
Conclusions: Managing Impacts to Vegetation and Habitat .............................................278
Managing Impacts To Fisheries...........................................................................................278
Hatchery-raised Trout Releases .......................................................................................278
Salmon Smolt Releases / Runs ........................................................................................279
Nets / Weirs ....................................................................................................................279
Equipment Modification..............................................................................................280
Harassment..................................................................................................................280
Alternative Perches and Prey.......................................................................................281
Reduce Time Fish Spend in Nets / Weirs.....................................................................281
4) Conclusions: Managing Impacts To Fisheries..............................................................281
Survey Results ....................................................................................................................281
PART VI. POPULATION MODELING.................................................................................283
INTRODUCTION ..............................................................................................................283
DETERMINISTIC EXPONENTIAL MODELS .................................................................284
Basic Exponential Growth Models ..................................................................................284
Age-Structured Exponential Growth Models ...................................................................288
FINAL DRAFT Table of Contents xviii
Lake Ontario ...............................................................................................................288
St. Lawrence Estuary, Québec .....................................................................................289
Lake Champlain, Vermont...........................................................................................291
DETERMINISTIC LOGISTIC MODELS...........................................................................293
Basic Logistic Growth Models ........................................................................................293
Age-Structured Logistic Growth Models .........................................................................294
Great Cormorants in Europe................................................................................................295
CONCLUSIONS ................................................................................................................296
PART VII. RESEARCH NEEDS............................................................................................297
DISTRIBUTION INFORMATION.....................................................................................297
DEMOGRAPHIC INFORMATION ...................................................................................297
DIET..................................................................................................................................298
IMPACTS TO FISHERIES AND AQUACULTURE..........................................................298
IMPACTS TO FLORA AND FAUNA................................................................................299
MANAGEMENT TECHNIQUES.......................................................................................299
ECONOMIC IMPACTS .....................................................................................................300
LITERATURE CITED...........................................................................................................301
APPENDIX I. PERCEIVED IMPACTS ASSOCIATED WITH DOUBLE-CRESTED
CORMORANTS.....................................................................................................................325
Survey Response.................................................................................................................325
Impacts to commercial / sport fish (reported by wildlife biologists, etc.)..........................325
Impacts to commercial / sport fish (reported by fisheries biologists) ................................326
Impacts to Aquaculture ...................................................................................................327
Impacts to Vegetation......................................................................................................328
Impacts to avian species ..................................................................................................329
Impacts to other fauna .....................................................................................................330
Damage to Private Property.............................................................................................330
Disease Transmission......................................................................................................331
Impacts to Water Quality.................................................................................................331
Impacts to Public Health .................................................................................................332
APPENDIX II. CURRENT RESEARCH................................................................................337
APPENDIX III. LIST OF COMMON AND SCIENTIFIC NAMES .......................................343
Birds ..................................................................................................................................343
Fishes.................................................................................................................................344
Mammals ............................................................................................................................346
Arthropods.........................................................................................................................347
Other Invertebrates..............................................................................................................347
Plants.................................................................................................................................347
APPENDIX IV. LIST OF ABBREVIATIONS .......................................................................349
APPENDIX V. LIST OF PERSONAL COMMUNICATIONS...............................................351
FINAL DRAFT Introduction 1
INTRODUCTION
Background Information
Since the 1970s, numbers of Double-crested Cormorants (Phalacrocorax auritus)
(DCCO) have increased significantly in many regions of North America. A variety of problems,
both real and perceived, have been associated with these increases, including impacts to
aquaculture, sport and commercial fisheries, natural habitats, and other avian species. Concern is
greatest over impacts to sport and commercial fishes, and to aquaculture. Impacts to the latter
appear more significant, but studies thus far have not determined the magnitude of these impacts.
The DCCO is protected in the United States by the Migratory Bird Treaty Act (MBTA)
of 1918, as amended (16 U.S.C. 703-712). DCCOs were added to the list of protected species (50
Stat. 1311, T.S. No. 12) in 1972 following amendment of the U.S. Convention with Mexico (23
U.S.T. 260, T.I.A.S. 7302). DCCOs are not included in the U.S. Convention with Great Britain,
acting on behalf of Canada for the Protection of Migratory Birds (39 Stat. 1702 T.S. No. 628 ),
and are therefore not protected by the Canadian Federal Government, although they receive
protection by provincial governments. In addition to U.S. federal protection, DCCOs may also
receive some protection at the state level.
Due to various concerns, legal actions have been taken to control DCCO numbers in the
U.S. In March, 1998, USFWS established a depredation order (Trapp 1998) in response to
complaints from catfish farmers and baitfish dealers. This order allows those engaged in
commercial aquaculture to shoot cormorants without a federal permit at freshwater aquaculture
premises or state-operated hatcheries in Minnesota and 12 southeastern states. Cormorant control
programs also exist in individual states to control numbers to reduce their impacts to island
vegetation and other colonial waterbirds, but these can only be carried out under the terms of a
federal permit.
Illegal actions to control cormorants have also occurred. In the summer of 1998,
frustrated fishermen who believed cormorants were responsible for game fish declines in the
eastern basin of Lake Ontario illegally shot an estimated 1500 – 2000 Double-crested
Cormorants on Little Galloo Island, Lake Ontario. In 1999, the State of New York requested a
depredation permit to reduce cormorants on eastern Lake Ontario primarily for the purpose of
controlling their predation on smallmouth bass, a popular sport fish. The State’s secondary
concern was to limit cormorant competition with other bird species. The USFWS issued a permit
to prevent all reproduction through egg oiling on Little Galloo Island, and to continue nest
destruction efforts on other islands on Lake Ontario to benefit other bird species and their
habitats. In Canada, legal and illegal control for similar reasons has also occurred.
A resolution was signed by George W. Bush, then the governor of Texas, asking USFWS
to evaluate the economic impact of the DCCO on sport fishing and to “consider removal of the
Double-crested Cormorant from the protection of the Migratory Bird Treaty Act if national
economic losses warrant severe control methods to keep the cormorant population at a
manageable level within a given region.” Because of these strong concerns over potential
impacts associated with increases in DCCOs, the USFWS is preparing an Environmental Impact
Statement (EIS), and in conjunction with the U.S. Department of Agriculture / Wildlife Services
(USDA/WS) and state resource management agencies, plan to develop a national management
plan for the DCCO. This status assessment will serve as a primary resource to aid in the EIS and
development of the management plan.
FINAL DRAFT Introduction 2
The Status Assessment
To assess the status of the Double-crested Cormorant, we reviewed natural history,
population data and trends, diet studies, cormorant impacts to vegetation and colonial waterbirds,
cormorant control, population modeling, current research needs and future research priorities.
Much of this information was obtained through a review of published literature, literature in
press, and unpublished reports. In addition to summarizing information, we evaluated some
studies (mainly those which attempted to determine impacts of DCCO predation) for accuracy
and thoroughness. Similar increases in numbers of the Great Cormorant (Phalacrocorax carbo)
(GRCO) have occurred in Europe, and similar conflicts with human interests have arisen.
Because the DCCO and the European GRCO are ecological counterparts, and much significant
research has been conducted on the latter species, we incorporated relevant insights gained
through study of the GRCO.
We also conducted two surveys to obtain information. We first conducted a DCCO
survey to obtain data on breeding, wintering and migration numbers and distributions, population
trends, and current research, monitoring and management / research priorities; and to obtain
information on the perceptions of impacts to natural resources and damage problems associated
with DCCOs in each state / province. We sent this survey to agency (both government and non-government)
wildlife biologists, university and museum biologists, and to USDA/WS personnel
in areas where problems associated with cormorants are high. Survey recipients were asked to
identify a fisheries biologist to whom we could address specific questions regarding impacts to
sport and commercial fish. We then conducted a second survey with fisheries biologists to obtain
more information on perceptions of impacts to fish and to gain a wider perspective on this
problem. Where questions arose regarding information provided in the surveys, we made follow-up
calls and tried to resolve unclear issues.
FINAL DRAFT Part I. Natural History 3
PART I: NATURAL HISTORY OF THE DOUBLE-CRESTED
CORMORANT, PHALACROCORAX AURITUS
[This section is a brief summary based largely on the Birds of North America species
account of the Double-crested Cormorant (Hatch and Weseloh 1999). Parts of some sections
(breeding and winter range) come directly from this account; for a full treatment of the natural
history of this species and for specific data sources the reader should see account. Other
literature and information sources were also occasionally included.]
GENERAL INFORMATION
Taxonomy
The Double-crested Cormorant (DCCO) is in the order Pelecaniformes, family
Phalacrocoracidae. Worldwide, between 30 – 40 species of cormorants are recognized. In earlier
taxonomies most species were placed in the single genus Phalacrocorax; recent accounts favor
dividing the family into two main groups, the typical cormorants and the strictly maritime ones,
the “shags.” In North America, there are six species of cormorants, including the Double-crested;
these include Great Cormorant (P. carbo), Neotropic Cormorant (P. brasilianus) [formerly called
Olivaceous Cormorant (P. olivaceus)], Brandt’s Cormorant (P. penicillatus), Pelagic Cormorant
(P. pelagicus), and Red-faced Cormorant (P. urile). The range of the Double-crested overlaps
those of Brandt’s and Pelagic Cormorants on the Pacific Coast, from southern Alaska to the Baja
Peninsula; the Red-faced Cormorant mostly along the coast of southern Alaska; the Neotropic in
Texas, Louisiana, and w. Mexico, mainly Sinaloa; and the Great Cormorant along the east coast,
from Newfoundland to Florida. The Double-crested is most closely related to the Neotropic
Cormorant and is in the same subfamily (genus) as the Great (carbo) and Brandt’s (penicillatus)
cormorants. The other subfamily includes species provisionally named as shags.
Five subspecies of the Double-crested Cormorant have been described, based on size and
crest characters: 1) P.a. auritus (Northern Double-crested Cormorant), most numerous, breeds
widely in the interior and on the northeast coast; 2) P.a. cincinatus (formerly White-crested or
White-tufted Cormorant) occurs in Alaska; 3) P.a. albociliatus (formerly Farallon Cormorant)
breeds on Pacific Coast and inland, possibly to New Mexico, Utah and Montana; 4) P.a.
floridanus (formerly Florida Cormorant) is resident in Florida and Caribbean; 5) P.a. heuretus
(no common name) is resident on San Salvador I., and possibly other islands of the Bahamas (see
figures in Palmer 1962, p. 331; Johnsgard 1993, Fig. 51, p. 200). Characteristics of birds
breeding in Mexico have not been established. Banding recoveries suggest that little mixing
occurs across the Rocky Mountains.
Physical Description
The DCCO is a medium to large (typical length 70 – 90 cm, body mass 1.2 – 2.5 kg) dark
waterbird. Cormorants have elongated bodies, moderately long necks and bills, long wedge-shaped
tails, and totipalmate feet. Resting birds often hold wings in a spread-wing posture,
thought to aid in drying wet feathers. Distinctive characteristics of the DCCO include brilliant
turquoise eyes, a golden throat pouch, elegantly patterned back feathers, and for a short time
prior to breeding season, a cobalt-blue mouth. Plumage is black or dark brown with a dull
FINAL DRAFT Part I. Natural History 4
greenish or bronze gloss. There is considerable variation in body size, and in color and size of
crest. Colors of eye-lid may vary geographically or individually from blue to orange. The genus
name, Phalacrocorax, is Greek for “bald-headed raven”; the species name, auritus, is Latin for
“eared” and refers to the crests above the eyes, feathers which are fully developed only for a
short time early in the breeding season.
Range
Widely distributed in North America, but not elsewhere. Vagrants have been reported
from England 1989, and the Azores in 1991.
Breeding Range.
Five major breeding zones have been defined:
1) Alaska
2) Pacific Coast
3) Canadian and U.S. Interior
4) Atlantic Coast
5) Florida and the western Caribbean
The five breeding zones are partly reflected in subspecific designations. Populations of
P.a. auritus have expanded and contracted from two areas: freshwater breeders in the Prairie
provinces, and maritime birds in the northwest Atlantic. The other four subspecies are
concentrated in Alaska (P.a. cincinatus); along the Pacific Coast (P.a. albociliatus); in Florida
and the Caribbean (P.a. floridanus); and in the Bahamas and Cuba (P.a. heuretus). The Alaskan
population breeds at Nunivak Island., and se. Bering Sea from e. Aleutian Islands. to the
southeast coast, including Kodiak Island. The Pacific Coast population breeds between s. British
Columbia and Sinaloa, Mexico; most breeding is coastal, though some occurs inland. The
Canadian and U.S. interior population breeds from n. Alberta through central Ontario; James Bay
and sw. Québec, south to central Utah, central Colorado, w. central Nebraska, se. South Dakota,
w. central Minnesota, ne. Iowa, central Wisconsin, and n. Lower Peninsula of Michigan; range
extends west to sw. Idaho and east along GL to lower St. Lawrence River. Also breeds locally in
other areas within the interior (see Hatch and Weseloh 1999). On the Atlantic Coast, breeds from
Newfoundland to New York, and small numbers elsewhere (see Hatch and Weseloh 1999). Most
breeding is coastal. In the southeast, resident in Florida, and occurring locally in small numbers
along Gulf Coast to Texas. Scattered residents in Cuba, and uncommon resident in Bahamas.
Also nests on coast of Yucatan Peninsula and n. Belize. Recent expansion has led to blurred
boundaries for Zones 3, 4 and 5. For further discussion and delineation of the five breeding
zones, see Part II, Biologically Relevant Units and Organization of Data.
Winter Range.
On the Pacific Coast and Alaska, chiefly resident, though some dispersal occurs. Birds
breeding in the interior and on the Atlantic Coast are strongly migratory, and most winter
coastally from North Carolina to the western Gulf of Mexico. Also winters along the Gulf of
Mexico from Tamaulipas south to the Yucatan Peninsula and Belize. Inland, significant numbers
occur at lakes, rivers and impoundments; regularly winters inland from Atlantic Coast along
major rivers to se. Pennsylvania, central Maryland, and throughout lower coastal plains of
FINAL DRAFT Part I. Natural History 5
Virginia and the Carolinas. Also winters inland from Gulf Coast (especially along the
Mississippi and other rivers) north to central Georgia, n. Tennessee, sw. Indiana, s. Illinois, n.
Arkansas, e. Kansas, eastern half of Oklahoma, Texas (except for Panhandle). Small numbers are
increasingly observed in coastal New England, the Maritimes, the Great Lakes, and the prairies.
Other populations breeding in Florida, Mexico, Cuba, and the Bahamas are resident.
Habitat
Breeding Season Habitat Requirements.
Colonies require sites safe from ground predators and close to feeding areas (usually < 10
km). Ponds, lakes, slow-moving rivers, lagoons, estuaries and open coastlines are utilized. Where
available, selects small rocky or sandy islands. May also use artificial sites such as bridges,
wrecks, abandoned docks or purpose-built towers. Though nests on ground or in trees, ground
nesting may be the ancestral and preferred habit; tree nesting may be response to predators. Trees
used for nesting are usually standing in or near water, on islands, in swamps, or at tree-lined
lakes. Where predators are present, depends on flooded snags or live riparian trees. Also nests on
emergent vegetation in marshes. In all seasons requires suitable places for nighttime roosts and
daytime resting or loafing. Roosts and resting places are often on exposed sites such as rocks or
sandbars, pilings, wrecks, high-tension wires or trees near favored fishing sites.
Winter Habitat Requirements.
Largest numbers of wintering birds occur along the southern coasts, and require similar
characteristics in feeding, loafing and roosting sites as when breeding. Cormorants wintering
along the North Carolina coast roost in or near the inlets on sandbars in dense flocks of up to
5,000 – 10,000 birds, or more, so that the sandbars are often blackened. They roost so close they
are nearly touching each other. Sandbars or shoals chosen are high enough that they are not
underwater at high tides. During the day, birds feed in large flocks, often numbering several
thousands, in the inlet, sounds, and inshore ocean. Very little roosting is in trees, but some roost
on channel markers, pilings, etc. Birds also rest on sandbars and markers during the day. They
are often seen flying to sandbars, and then flying out to sea in the afternoon; at dusk they return
to the sandbars (H. LeGrand, pers. comm.). Around aquaculture sites winter numbers are
increasing, and in Mississippi many that winter near catfish farms roost in isolated cypress
swamps. There may be differential selection between sexes in winter habitat use; in Mississippi,
males predominated in samples shot at inland roosts (Glahn et al. 1995), and in Texas females
predominated in a sample collected in Houston Ship Channel (King et al. 1987).
BIOLOGY
Migration
Atlantic Coast and interior nesting birds are highly migratory; all age groups migrate. In
other areas, the species is mostly resident within the breeding range and migratory habits are
poorly understood. More northerly populations breeding in Alaska may migrate south to s.
British Columbia and Washington.
Migrants wintering in the Gulf of Mexico begin heading northward as early as February.
Along the eastern seaboard they follow the Atlantic coastline; in the interior they follow river
systems and fly overland. Earliest individuals reach Oklahoma and Virginia by 4 – 5 March;
FINAL DRAFT Part I. Natural History 6
Massachusetts, the southern Great Lakes, Minnesota, S. Dakota, and s. Idaho by late Mar – early
April; upper Great Lakes by early to mid April; more northerly destinations, e.g., the prairie
provinces, coastal Maine, are not reached until mid-late April. In British Columbia, migration
occurs mid-Mar to late May, peaking late April – early May. Analysis of banding recoveries
indicates that first-year birds return north about a month later than older birds. Recently, some
birds have been remaining year-round and breeding along the Gulf Coast.
Autumn migration is essentially the reverse of spring. Earliest autumn migrants appear to
reach wintering grounds faster than they reached breeding grounds in spring. Migration
underway on the Canadian prairies by mid-late August, at which time first migrants are arriving
on the Upper Mississippi River, Ohio, Massachusetts, Virginia and Texas. By September,
migration well underway throughout range. Massachusetts, Rhode Island and Minnesota report
peak numbers of transients in October. Most banded migrants recovered in Oct and Nov. Along
coastal New England, bulk of migration observed 25 September – 17 October. Individuals arrive
on winter grounds September to November. Some fly across open ocean.
Inland, migrates in small flocks, < 50 – 100; along coasts, near shore flocks may consist
of 1000s of birds. Migratory flight begins soon after dawn and continues all day, with flocks
observed flying late in evening.
Reproduction
Usually forms dense breeding colonies (100s – 1000s); solitary nests very rare. Often
nests in mixed-species colonies. DCCOs are believed to have extreme fidelity to colony sites.
Many young first breed at natal colonies, and philopatry to proximity of natal colony is probable;
in banded birds recovered in June that were at least 3 years old, the median distance to natal
colony was only 25 km.
In British Columbia, most DCCOs began breeding as 3 year olds (in their 4th summer).
However, a small portion of color banded birds first bred as 1 year olds (4.7 %) and 2 year olds
(16.5 %). Immatures (1 and 2 yr olds) are present on breeding colonies by June.
Ground nesting cormorants typically nest on low-lying rock islands or reefs, away from
or with sparse vegetation. Arboreal nesters use evergreen and deciduous trees, initially alive but
killed within 3 – 10 years due to guano deposition over time. Nesting substrates include
limestone and granite bedrock, large boulders or fallen trees, gravel, cobbles, beach ridges, soil
or standing trees. Possibly usurps attended or unoccupied nests of some herons and egrets. Nest
characteristically includes finger-sized sticks and other bulky items collected from diverse
locations, some several kilometers away. Along seacoasts seaweed forms the usual nest-base;
flotsam and jetsam often incorporated, as are parts of dead birds. Lining is usually of grass,
rootlets and similar materials. As breeding season progresses, the nest receives pebbles and
bones from pellets, and a coating of guano on the outside that seals the nest together. Many nests
are used repeatedly and built upon each year, becoming tall turret-like structures; some reach
heights of 2 m or higher. Tree nests are usually much smaller but have a deeper inner cavity than
ground nests. Ground nesters defend small area around nest within beak range (0.38 m).
Reported inter-nest distances vary widely, from 3.3 – 220 cm.
Double-crested cormorants lay between 1 – 7 eggs. Average clutch size ranges from 2.7 –
4.1; modal clutch size is four. First eggs are laid 2 – 4 weeks after arrival to breeding colonies. In
Ontario, egg laying occurs late Apr to Aug; in the St. Lawrence River estuary from about 10
May to Jul. Eggs are cylindrical ovate in shape. Shells are pale blue and unmarked, but
pigmented layer often obscured by variable outer calcite cover that is initially white and porous.
FINAL DRAFT Part I. Natural History 7
Calcite layer has chalky texture and gives irregular surface. Egg mass is small compared to other
seabirds; reported averages range from 44.9 – 46.5 g, 2.7 % of adult mass. Eggs are laid daily or
on alternate days. Following loss of complete clutch, relaying is frequent, occurring within 19
days. However, successful second broods are rare. Eggs are extremely cold tolerant, but
vulnerable to heat stress.
Incubation begins gradually, reaching a maximum intensity when third egg is laid. Eggs
are incubated by lying on top of warm webs of feet; abdomen and breast are lowered onto them.
Duration of incubation varies from 25 – 28 days. Both sexes incubate, but the female does more
during the first half of the incubation period. Changeovers occur at intervals of 1 – 3 hours.
Hatching occurs asynchronously, but intervals between eggs typically less than 1 day.
Chicks are altricial, and barely able to move at hatching. Eyes open at 3 – 4 days; egg
tooth drops off between 4 – 7 days. Down appears within about one week. Thermoregulatory
ability not complete until 2 weeks. Young are cared for equally by both parents. Brooding is
nearly continuous for the first 12 days, but ceases with the appearance of down and effective
endothermy. Young will remain in ground nests for about 28 days if undisturbed (though have
walked from ground nests as early as 21 days when approached by humans or predators). Young
in tree nests or in nests on cliffs may remain there until they are able to fly, which occurs
between 6 – 8 weeks of age. Diving ability develops and plumage is complete at about the same
time. Young are completely independent of parents at 10 weeks of age.
Annual reproductive success varies within and among colonies, but hatching success
usually 50 – 75 %; fledging success 1.2 – 2.4 young / nest or 74 – 95 %. Chick loss from
hatching to fledging is often low, e.g. 5 % in coastal B.C. All figures much lower for DDE-contaminated
populations. In St. Lawrence estuary, reproductive success parameters lower for
late nesting cormorants (June) compared to early nesters (May). Average lifetime production
(lifetime reproductive success) calculated for birds breeding on Mandarte I., B.C. was 3.28
young (van der Veen 1973).
Foraging Ecology
The DCCO is an opportunistic, generalist feeder, preying mainly upon abundant, easy-to-catch
fish species. Usually slow-moving or schooling fish, ranging in size from 3 – 40 cm but
commonly < 15 cm, are taken. The DCCO appears to be strictly diurnal in its feeding habits, and
usually forages in shallow water (< 8 m) within 5 km of the shore. The prey of Atlantic birds
suggests that they are more likely to feed at the bottom of the water column, while that of Pacific
and inland birds suggests that they feed in mid-water. Cormorants respond rapidly to high
concentrations of fish and often congregate where fish are easily caught, such as put and take
lakes, stocking release sites, aquaculture ponds, dams, and other areas where fish are
concentrated.
To capture food, cormorants dive from the surface and pursue prey underwater. Prey is
grasped in the bill and may be swallowed underwater. If the prey is large or difficult to handle,
such as eels, flounders or spiny fish, it may be swallowed at the surface after first being shaken
and hammered on the water. Prey is sometimes thrown in the air, caught and swallowed head-first.
Frequently forages individually, but readily gathers to form feeding flocks (tens to
hundreds). When feeding on schooling prey, sometimes loosely-coordinated foraging flocks are
formed. This behavior has been observed most often in the breeding season and in late summer /
early fall. Bottom-feeding is usually solitary.
FINAL DRAFT Part I. Natural History 8
Diet
Information on the diverse diet of this species has been recorded from as early as 1835,
when Audubon (1835) observed that the food of Double-crested Cormorants consisted of
“shrimps, lents, capelings, codlings, and other fishes”. In general, primary prey are forage
species of little or no commercial value, but cormorants will take advantage of abundant species
in the right size range. Over 250 fish species from more than 60 families have been reported as
prey items. Occasionally, other aquatic animals, such as insects, crustaceans and amphibians are
also taken. Diet is discussed specifically by region in PART III: DIET.
Longevity, Survival and Mortality
Oldest banded bird reported was 17 years, 9 months of age; however, wear and loss of
aluminum bands is likely to lead to underestimates of survival based on recoveries. For birds
banded as fledglings on Mandarte I., B.C., first year survival was estimated at 0.48, second-year
at 0.74, and subsequent annual survival of 0.85; mean adult life expectancy 6.1 year (van der
Veen 1973). Mortality factors for young birds include disturbance at breeding colonies, which
can result in large mortality of hatchlings from exposure, and of eggs and young by predation;
Newcastle disease can also cause significant mortality among young birds. Adult and large chick
mortality factors include predation by Bald Eagles (Haliaeetus leucocephalus), entanglement in
fishing gear, and shooting. Rates of mortality due to predation were not available. Fishing gear is
reported as a major cause of death. Of birds banded in the Great Lakes between 1928 – 1995, 9
% of those recovered were shot; of band recoveries in Texas, 17 % were shot.
FINAL DRAFT Part II. Population Data and Trends, Introduction 9
PART II. POPULATION DATA AND TRENDS
INTRODUCTION
Most population and trend data were obtained through the survey sent to wildlife
biologists. We also utilized published literature, breeding bird atlases, unpublished reports and /
or papers in press regarding trends and / or the history of cormorants in specific states and
provinces. Survey recipients were asked to provide data on breeding, wintering, and migration
concentrations. For breeding birds we asked recipients if cormorants were recent (1972 or later)
breeders in their state / province, and to provide number of pairs, ownership of site (if colony >
300 pairs), and any productivity estimates. For wintering populations, we asked for number of
birds and site ownership information (if concentrations > 1000 birds). For migration
concentrations, we asked for peak migration times and numbers and land ownership for
significant stopover sites (concentrations of > 500 birds). We also telephoned and e-mailed many
individuals who provided us with data to get more precise information regarding numbers and
trends in their states and provinces. Individuals completing surveys sometimes provided possible
reasons for population increases; these were incorporated into the state and provincial profiles.
Breeding Birds
Biologically Relevant Units and Organization of Data
Five main zones for breeding Double-crested Cormorants in North America were defined
by Hatch and Weseloh (1999) and these largely correspond to subspecies ranges (see Part I:
Breeding Range.). In an effort to present biologically relevant information on biological units, or
“populations”, we organized and presented population data based largely on these population
zones, with a few modifications. In North America, populations of breeding cormorants occupy
five main areas: Alaska (Zone 1); the Pacific Coast, from British Columbia to Mexico (Zone 2);
the Canadian and U.S. interior (Zone 3); the Southeastern U.S (Zone 4), and the Atlantic Coast,
from Newfoundland to New York (Zone 5) (Figure 1). For the most part, these zones are
geographically or biologically (in terms of subspecies) distinct. Data were organized in this
manner to aid and encourage individuals to view the biological unit, rather than political or
organizational boundaries, when developing management plans.
These zones are thought to reflect fairly distinct breeding populations, but recent DCCO
expansion and re-colonization has blurred the boundaries between these zones (Hatch and
Weseloh 1999). Border states between interior and southeastern populations, interior and western
populations, and southeastern and northeast Atlantic populations (e.g., Texas, New Mexico,
Idaho, inland southern states, the Carolinas), qualify as “gray areas.” In these areas, it is not
possible to determine precisely which zone breeding cormorants belong to; therefore, it was not
possible to delineate absolute boundaries for each zone. However, because we were interested in
presenting as much biologically relevant information as possible, and birds breeding in “gray
areas” were in relatively small numbers, we determined the most likely population zone for each
colony documented between 1970-2000 (Figure 1). These determinations were based on the best
information available on subspecies range (Palmer 1962; Johnsgard 1993; Hatch and Weseloh
1999), and consideration of logical geographic units.
Additionally, to help with decisions for birds breeding in inland southern states (AR, OK,
TN), southeastern coastal states (NC, SC), and southwestern states (TX, NM), we also
FINAL DRAFT Part II. Population Data and Trends, Introduction 10
considered winter distribution, and the fact that birds breeding in many of the southern states are
likely year-round residents, while birds from neighboring Zones 3 and 5 are migratory. The
winter range in North America is quite distinct and occupies two main areas: in the eastern and
central portion of the continent, the species winters mostly from Texas to Florida along the Gulf
Coast, and along the Atlantic Coast through the Carolinas and Virginia; in western North
America, the species winters mostly along the Pacific Coast, from southern British Columbia to
Baja California Sur and the Gulf of California. Large numbers also winter inland in Texas,
Oklahoma, and Arkansas. The large number of birds wintering in the eastern and central portion
is a fairly easily defined, succinct, and very important unit in terms of human-cormorant
conflicts. Therefore, it made the most sense to keep the southern states together and acknowledge
the possibility that the small number of birds that breed in the boundary states of this zone may
in fact be part of the interior or northeast Atlantic populations. For birds breeding in Idaho,
where some have been found to possess characters intermediate between subspecies P.a.
albociliatus and P.a. auritus (Burleigh 1972), suggesting that mixing occurs at the “boundaries”
of Zones 3 and 5, we decided to rely on Burleigh’s (1972) conclusion that Idaho birds examined
came closer to P.a. auritus than to P.a. albociliatus; thus we grouped Idaho birds with the
interior population.
Therefore, when viewing these population zones, we suggest that special attention be
paid to boundaries, especially “gray areas”. Extent and areas of overlap need to be more closely
defined, especially if management actions are considered at a regional level or potential impacts
to a population are under review. More detail is presented on population organization in the
individual summaries for the zones; the problem is also addressed in PART VII. RESEARCH
NEEDS.
Colony Locations
Survey recipients were asked to provide latitude-longitude coordinates for each colony
location known since 1970. If these data were not available, we made follow-up calls and tried to
determine approximate locations. We were able to obtain relatively good location data for most
colonies and have included maps showing the distribution of colonies in each population zone
during the last 30 years (Figure 1). In Zone 1 we show distribution of all known colony sites. In
Zone 2 we show distribution of all known colony sites with the exception of interior California.
D Shuford at Point Reyes Bird Observatory will supply a map for this area (D. Shuford, pers.
comm.). In Zone 3, we were unable to obtain complete data for the following states and
provinces: for Iowa, we were unable to obtain coordinates for 4 of 10 known colonies; for
Minnesota, we were unable to obtain coordinates for 7 of 75 known colonies; for Alberta,
Montana, North Dakota and Colorado we were unable to obtain any precise information. In Zone
4, we obtained coordinates for all known breeding colonies, with the exception of Florida, where
coordinates were provided for “most” cormorant colonies in the state. In Zone 5, we obtained
coordinates for all known colonies except for 5 of 12 on Prince Edward Island, and 2 of 9 in
Newfoundland.
Maps showing distribution of active colonies by size at time of last survey are included
for states and provinces for which we have the appropriate data in the five breeding zones. In
Zone 4, the only state for which we produced this type of map was South Carolina. With the
exception of Florida, for which the necessary data were not available, most states in this zone
have marginal breeding numbers with few and relatively small colonies.
FINAL DRAFT Part II. Population Data and Trends, Introduction 11
Trends
To determine trends in breeding numbers, we asked survey recipients to supply data on
all known breeding colonies over 4 time periods: 1970, 1980, 1990, and 1998. Not surprisingly,
many states and provinces did not have data for these specific years, so we asked for data from
censuses conducted around these years. If that was not possible, we asked for whatever data were
available. If complete counts were made for at least two years we generated population trends
(Table 1, Table 4,Table 5,Table 6; see also figures in state and province summaries). To estimate
percent average annual rate of change in number of pairs, we calculated er-1, where
r = ln (recent count) – ln (earlier count) and er = l
years between counts
(Smith 1992). For figures we used all complete data available. For eastern and central North
America (Zones 3, 4, 5), when no complete counts were available, we examined colonies within
a state or province that had > 2 years of data and compared the number of colonies that had
increased at last count with the number that had decreased.
We also provided estimates of size of regional breeding populations for Zones 3 – 5
(Table 4, Table 5, Table 6), based on latest totals of breeding pairs in each state and province that
made up the regional population. We caution that estimates for each state and province may
utilize different survey methodologies, and may not all be obtained in the same year, thus
regional estimates should be considered rough approximations at best. For Zones 1 and 2,
comprehensive data were not available for an estimate of the entire region.
Because data were incomplete for many areas and / or time periods we did not attempt to
calculate a rate of change for the continental or even regional populations. However, Tyson et al.
(1999) reported that the number of DCCOs nesting in the U.S. and Canada increased about 2.6 %
annually from 1990 through 1994. This trend was based on data that ranged from recent
complete counts to conjectures based on old or incomplete information. While the mean percent
annual change in the number of nesting pairs in those states and provinces that had recent
complete counts was 16.2 %, the majority of states and provinces did not have recent complete
counts, and thus Tyson et al. (1999) suggest that these rates of change be used with caution.
Wintering Birds
Locations and Winter Distribution
Survey recipients were asked to provide latitude-longitude coordinates for all known
wintering locations, including feeding and roosting sites where major concentrations (> 1000
birds) occurred. However, the majority of U.S. and Mexican states, and Canadian provinces do
not conduct surveys for winter birds, and could not supply latitude-longitude information for
DCCO wintering areas. Thus it was not possible to create a precise winter distribution map. We
suggest that Hatch and Weseloh (1999) be consulted for winter distribution across the continent.
To provide general information on wintering cormorants, we utilized data obtained through
annual Christmas Bird Counts. A few states (AL, AR, and MS) provided locations for winter
roosts (see below).
Use of Christmas Bird Count Data
The most comprehensive data for wintering DCCOs in the U.S. and Canada were
obtained from annual Christmas Bird Counts (CBCs). Annual CBCs have been the only method
FINAL DRAFT Part II. Population Data and Trends, Introduction 12
to consistently monitor winter birds on a broad scale. While CBCs are not undertaken to produce
precise population estimates, they provide a useful index of population trends and valuable
information on winter occurrence. For some states, CBC data provided the only information
available about wintering cormorants. Therefore, for states with major wintering numbers (> 100
birds overwinter) we obtained CBC data (locations and numbers) collected between 1984 – 1997
for Zone 4 and 1984 – 1998 for Zones 1, 2 and 5. We also obtained data collected between
1984-1998 for states in Zone 3 that border Zones 2,4 and 5. CBC data were accessible for 1984-
1998 at the time of our assessment (Sauer et al. 1996). For each state and province, we included
all CBC sites where > 100 birds were counted at least once during the period under consideration
(see figures in state summaries). These sites are shown in Figure 2.
Wintering Numbers and Trends
In general, data on numbers of over-wintering birds were limited and it was not possible
to determine trends. The information we provide on wintering trends is based mainly on
“qualitative” assessment of apparent changes in numbers counted during CBCs. For some states
(AL, BC, CA, FL, GA, KS, LA, ME, MA, MI, NJ, NY, NC, OK, OR, TN, TX, VA, WA) Sauer
et al. (1996) analyzed CBC data (1959 – 1988) for trends, included in this assessment. CBC data
(1989 – 1999) have not been analyzed, but are included to provide a broad picture of recent
changes in winter numbers. CBC data should be interpreted cautiously, for a number of reasons
(e.g., number of counts is not always consistent within states and provinces; observer skill level
varies).
Limited survey data from Mississippi were included to provide a broader picture of
recent changes in wintering numbers in this state, mainly in the Mississippi Delta region.
Additionally, limited survey data from Alabama, Arkansas and Mississippi data were
incorporated to compare CBC site distribution with known night roosts located by Wildlife
Services surveys (Figure 51). Additionally, comments on perceived trends from biologists and
other individuals or sources providing us with information on winter numbers were incorporated.
At this time, it is not possible to provide an estimate for winter population size because data are
incomplete. However, this may be an important estimate for management actions geared toward
reducing impacts on the breeding and wintering grounds (see Non-breeding Birds and Total
Population Size, below, and PART VII. RESEARCH NEEDS).
Migrant Birds
Migration / Stop-over Sites
Survey recipients were asked to provide latitude-longitude coordinates for all major (>
500 birds) migration stop-over sites. For most states, migrant numbers were very difficult to
obtain because often no information was available, or migrants could not be distinguished from
wintering birds. Some states provided general information about where concentrations of migrant
birds were regularly seen, but because data were so limited we did not map them.
Trends
We provide limited information for a few states and provinces on trends in numbers of
migrants in the specific state/provincial accounts.
FINAL DRAFT Part II. Population Data and Trends, Introduction 13
Non-breeding Birds and Total Population Size
We did not obtain information on non-breeders, which include sub-adult and adult non-breeding
birds. Currently there is very little information about this segment of the population,
and without this information, it is not possible to estimate the continental population size with
any degree of precision. Life-tables have not been established for any of the breeding
populations, age of first breeding can vary, and year-to-year differences in non-breeding by
adults can be significant (Hatch 1995; Hatch and Weseloh 1999). At various times, different
investigators (e.g. Lewis 1929; McLeod and Bondar 1953; Price and Weseloh 1986; Watson et
al. 1991) have estimated that a range of 1.0 – 4.0 non-breeding cormorants per breeding pair
determines the size of the non-breeding segment of North American cormorant populations. In
recent estimates of population size, these estimates have been applied to breeding numbers.
Hatch (1995) estimated the continental breeding population at > 360,000 pairs from data
obtained mainly in the 1980s and early 1990s; applying the range of estimates for non-breeders,
he estimated a continental population size of 1 – 2 million birds. Tyson et al. (1999) estimated a
minimum of 372,410 breeding pairs in the U.S. and Canada, based on data collected between
1975 – 1997 (most data were collected 1994 – 1997). Applying the range of estimates above for
non-breeders, Tyson and colleagues conservatively estimated that > 1 million individual DCCOs
currently occur in the U.S. and Canada. Hatch (1995) noted that totals are imprecise because
large regions have been poorly studied and because some of the largest populations are the least
well known (e.g., Manitoba, Mexico). Additionally, review of the estimates by Lewis (1929),
McLeod and Bondar (1953) and Price and Weseloh (1986) indicated estimates for non-breeders
were not based on detailed scientific study and rigorous data. Before estimates for this segment
can be made with any confidence, careful study and observations at specific colonies (e.g. older,
stable colonies vs. newer, rapidly expanding ones), as well as at locations without colonies where
non-breeders may aggregate, need to be undertaken (D.V. Weseloh, pers. comm.).
In addition to providing important information for total population size, an estimate of
non-breeders is also important for predicting population growth, and effectiveness and impacts
of potential management strategies. For example, at the large Oostvaardersplassen Great
Cormorant (GRCO) colony in The Netherlands, Van Eerden and Van Rijn (1997) estimated that
at least 40 % of birds present did not breed. Based on this estimate, they concluded that any
intervention in colonies aimed at reducing number of breeders is likely to allow non-breeders to
take over the empty territories, or disperse and colonize new territories.
For predicting appropriateness of management strategy, an estimate equally important to
total population size (e.g., regional, continental) is the proportion of birds utilizing various
habitats and resources. For example, Van Eerden and Van Rijn (1997) reported that fish farms
provide food for only about 0.5 – 1 % of the total population of European GRCOs in winter, and
thus managing on the local level was considered more appropriate. We do not have good
estimates for the proportion of the DCCO population utilizing aquaculture farms. For example,
Nisbet (1995) estimated that the number of DCCOs wintering in the Mississippi Valley is
probably equivalent to no more than five percent of the total interior population. However, this
estimate represents the number present at a given point in time rather than the total number of
birds moving through the region, which may be a larger proportion of the total population.
Dolbeer (1991) estimated from banding records that 120,000 birds might move through this
region during winter months. J. Glahn (pers. comm.) suggested that the number currently passing
though has probably more than doubled since Dolbeer’s (1991) estimate. There are two levels of
uncertainty surrounding these estimates. First, we cannot determine the proportion of the
FINAL DRAFT Part II. Population Data and Trends, Introduction 14
population the migrants represent because total population size is unknown. Second, because it is
not clear how many of these birds are overwintering and how many are simply passing through,
it is difficult to determine level of impact this portion of the population has on aquaculture.
Therefore, better estimates on the proportion of the population utilizing fish ponds and the
variation of residence time among birds present in the Mississippi Delta region are important for
decisions about whether to manage cormorants on a population or local level (see PART V.
MANAGEMENT OPTIONS).
FINAL DRAFT Part II. Population Data and Trends, Introduction 15
Figure 1. Distribution of Double-crested Cormorant (Phalacrocorax auritus) breeding
colonies in North America 1970 – 2000.
FINAL DRAFT Part II. Population Data and Trends, Introduction 16
Figure 2. DCCO Christmas Bird Count sites (1984 – 1998) where ³ 100 birds were counted
at least once.
FINAL DRAFT Part II. Population Data and Trends, Zone 1 17
ZONE 1: SUMMARY OF POPULATION DATA FOR ALASKA
Introduction
In Zone 1, the breeding population occurs entirely in Alaska, at Nunivak Island and the
southeast Bering Sea, and from the Aleutian Islands to the southeast coast, including Kodiak
Island. Inland it occurs to Lake Louise; the first nest in the Yukon Territory was confirmed at
Lake Laberge in 1998 (Hatch and Weseloh 1999). The westernmost limit occurs in the eastern
Aleutian Islands at Chuginadak Island (52°51’01” N, 169°49’41” W) (Carter et al. 1995).
Because DCCOs breeding in Alaska represent the entire P.a. cincinatus subspecies (Carter et al.
1995), the largest of the five subspecies (Hatch and Weseloh 1999), the summary of this
population zone is fully treated in the Alaska state summary, below.
Alaska
Summary of Population Data and trends
(Surveys completed by Don Dragoo, Alaska Maritime NWR, USFWS, Homer; and Shawn
Stephenson, Migratory Bird Management, USFWS, Anchorage.)
Breeding.
While we did not determine when DCCOs were first documented as a nesting species in
Alaska, it is clear that the species has been a long time breeder in the state (see Historical
Information, below). Between 1970 and 2000, the species was confirmed breeding at 126
colonies. Most breeding occurs along the southern coast and on the Aleutian Islands (Figure 4).
Because Alaska has not conducted statewide censuses of all colonies in the same year, the
number of breeding birds is not known. Additionally, most colonies have only been censused
once or not since the 1970s (Carter et al. 1992); therefore population trends are not available.
However, of 126 known colonies, 106 were counted at least once between 1970 and 2000; the
vast majority (93 %) were small, with < 100 pairs on average. Carter et al. (1995) estimated a
total of 2,811 pairs bred at 90 coastal colonies between 1970 and 1992, at which time comprised
12 % of the Pacific Coast population, defined by Carter et al. (1995) to encompass P.a.
cincinatus in Alaska, and P.a. albociliatus, breeding from southern British Columbia to Sinaloa,
Mexico. In the interior, colonies have not been adequately surveyed.
Numbers are thought to have declined since historical times, especially after introduction
of predators (see Historical Information, below). At present, oil spills threaten cormorants in
several areas, and human disturbance at breeding colonies remains a problem (Carter et al.
1995). On coastal islands, introduced predators [e.g., red and arctic fox, Norway rat, ground
squirrel (Spermophilus undulates), rabbit are being removed], which may enable former breeding
sites to be re-colonized by cormorants (Bailey 1993; Carter et al. 1995).
Winter.
In winter DCCOs occur, but no information was provided on wintering locations and
winter censuses are not conducted. Gabrielson and Lincoln (1959) reported the species was
resident in its breeding range, except possibly in the more northern areas, and that it was a fairly
common winter bird in southeastern Alaska. During CBCs conducted between 1984 – 1998, only
3 sites had > 100 DCCOs during at least one count, and fairly small numbers were observed
FINAL DRAFT Part II. Population Data and Trends, Zone 1 18
overall (Figure 3). The CBC data have not been analyzed by the Patuxent Wildlife Research
Center for trends during this period, and no trends are readily apparent from Figure 3.
Migration.
No information was available on migrants or migration in Alaska.
Historical Information
The DCCO is a long time resident and breeding species in Alaska. Remains of DCCOs
found in middens on Amchitka Island date back 2,650 years (Siegel-Causey et al. 1991). Midden
remains indicate DCCOs were formerly more abundant in the central Aleutian Islands, and
climate changes are thought to have probably reduced numbers in Alaska over time (Carter et al.
1995). While it has been suggested that the introduction of arctic foxes onto nesting islands
beginning in 1750 (Bailey 1993) probably greatly reduced breeding numbers (Carter et al. 1995),
it is not entirely clear how severely these introductions impacted DCCOs. In the late 1800s,
DCCOs were reported as abundant residents and breeders in the Near Islands in the western
Aleutians (Turner 1885), where foxes at this time were already present; thus whatever damage
foxes had done would already have been evident (V. Byrd, pers. comm.).
Land Ownership
No information provided.
Productivity
In 1998, productivity was monitored at Aiktak Island, and 1.8 chicks / nest fledged (Byrd
et al. 1999). On Duck Island, productivity was monitored in both 1998 and 1999, and was 0.0
and 0.13, respectively (Byrd et al. 1999; D. Dragoo, pers. comm.).
Figure 3. December estimates of DCCOs in Alaska based on CBCs, 1984 – 1998.
0
50
100
150
200
250
300
84 85 86 87 88 89 90 91 92 93 94 95 96 97 98
Year
Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 1 19
Figure 4. Distribution and size of active colonies in Alaska at time of last surveys (1970 –
1999).
FINAL DRAFT Part II. Population Data and Trends, Zone 2 21
ZONE 2: SUMMARY OF POPULATION DATA FOR THE PACIFIC COAST
Introduction
Breeding Information
In this zone, the breeding population occurs mostly along the coast from southern British
Columbia south to at least Bird Island, Sinaloa, Mexico; some birds may nest farther south
(Carter et al. 1995; Hatch and Weseloh 1999). Birds in this zone comprise the P.a. albociliatus
subspecies (Baird et al. 1884; Palmer 1962). Though most nesting occurs along the coast,
significant colonies also occur or have occurred inland in Washington, Oregon, California and
Mexico. Additionally, most birds breeding in Nevada, Arizona and New Mexico are presumed to
be members of this subspecies. However, Johnsgard (1993) suggested that birds breeding in
western Nevada at Pyramid Lake would seem to be more geographically affiliated with auritus,
but we have chosen to group the Nevada birds in population Zone 2 based on geographic
proximity to California and Arizona birds. In New Mexico, most of the breeding birds are
probably P.a. albociliatus, but a small number likely represent auritus (S. Williams, pers.
comm.). At this time we decided to include New Mexico birds in the Pacific Coast population
because auritus numbers appear to be marginal and data are still being analyzed (see New
Mexico profile for further detail). Birds breeding in Idaho and Utah have characters intermediate
between P.a. albociliatus and auritus, but these birds have been grouped in Population Zone 3
(see summary for Zone 3: Summary of Population Data for Canadian and U.S. Interior
Population for further information). Table 1 shows the status (Breeding, Wintering, Migrant) of
the Double-crested Cormorant throughout Zone 2.
No survey for this assessment was completed by several states in this region because
breeding data for much of the Pacific Coast have been compiled into one database by the
USFWS, Office of Migratory Birds and Habitat Programs-Pacific Region. We used this
database to acquire information on cormorant distribution and abundance.
For many states and provinces in this zone and Alaska, we cite percent of the Pacific
Coast population that breeding pairs in a particular area comprised, based on data from Carter et
al. (1995). They define the Pacific Coast population to encompass P.a. cincinatus in Alaska, and
P.a. albociliatus, breeding from southern British Columbia to Sinaloa, Mexico. Therefore, when
we give estimates of percent of the Pacific Coast population that pairs from a particular area
comprised, we refer to the Pacific Coast population as defined by Carter et al. (1995). Though
their definition of the Pacific Coast breeding population differs from ours, we cite Carter et al.’s
(1995) percent estimates to provide additional information on the number of birds occurring
within this region.
Along the coast, from British Columbia to Mexico, Carter et al. (1995) estimated that
approximately 21,849 pairs nested between 1968-1992. More recent estimates for the entire
region are not available, but significant changes occurred during the 1990s. Large increases were
documented along the coast in Oregon at the mouth of the Columbia River, and inland in
California at the Salton Sea; increases in these two areas account for most of the growth in the
region. Sharp declines were observed along coastal British Columbia and Washington colonies.
Because data are not available for birds breeding in significant portions of this region
(e.g., Mexico, some interior areas) and data collection has not been coordinated at a regional
scale, it is not possible to summarize trends for the population as a whole. Additionally, trends in
this region are strongly affected by apparent movements of birds during El Nino oceanographic
FINAL DRAFT Part II. Population Data and Trends, Zone 2 22
conditions, habitat loss at interior colonies, and use of artificial habitat in some areas (Carter et
al. 1995). With these habitat changes, distributional changes appear to be occurring; large
numbers of immigrants may account for much of the growth seen in particular areas (see
California and Mexico profiles). How these distributional changes affect overall population
growth for Zone 2 is not clear.
Winter and Migrant Birds
Though portions of the Pacific Coast and the interior provide wintering habitat for
DCCOs breeding in this region, no state- or province-wide counts of wintering DCCOs have
been conducted by state or provincial agencies monitoring cormorant numbers. Therefore, with
the exception of British Columbia, which reported that all breeding DCCOs are residential, no
precise estimates are available for winter numbers in this region. Additionally, while many of the
birds that breed in this zone are year round residents, some migration does occur, and it is
difficult to distinguish migrants from residents. Overall estimates of migrants in the region have
not been obtained.
Analysis of recent (1989 – 1998) CBCs conducted in this region has not been undertaken,
and trends are not readily apparent from data collected during counts conducted between 1984 –
1998 (Sauer et al. 1996) (Figure 5). However, CBCs indicate both coastal and interior areas of
California provide major winter habitat; numbers from California counts are much larger than
those reported in any of the other states (but see California profile for limitations of this data).
CBC data also suggest that coastal areas in British Columbia, Washington and Oregon provide
winter habitat for significant numbers of birds. In the interior, significant numbers are reported in
Arizona on larger reservoirs along the Colorado River, especially near Yuma, and in New
Mexico in the Elephant / Butte Caballo and Carlsbad areas (Figure 2).
Several areas in Mexico also provide significant winter habitat. The DCCO is commonly
found along both coasts of Baja Sur, and along the Gulf Coast of Tamaulipas and Campeche. It
is also a fairly common to common resident from the northwest Yucatan to northern Belize
(Howell and Webb 1995). However, no information is available on winter numbers in Mexico.
Historical Information
Carter et al. (1995) rev

Click tabs to swap between content that is broken into logical sections.

STATUS OF THE DOUBLE-CRESTED CORMORANT
(PHALACROCORAX AURITUS) IN NORTH AMERICA
PREPARED BY:
LINDA R. WIRES
FRANCESCA J. CUTHBERT
DALE R. TREXEL
ANUP R. JOSHI
UNIVERSITY OF MINNESOTA
DEPARTMENT OF FISHERIES AND WILDLIFE
1980 FOLWELL AVE.
ST. PAUL, MN 55108
USA
MAY 2001
PREPARED UNDER CONTRACT WITH *U.S. FISH AND WILDLIFE
SERVICE
*CONTENT MATERIAL OF THIS REPORT DOES NOT NECESSARILY
REPRESENT THE OPINIONS OF USFWS
Recommended citation: Wires, L.R., F.J. Cuthbert, D.R. Trexel and A.R. Joshi. 2001. Status of
the Double-crested Cormorant (Phalacrocorax auritus) in North America. Final Report to
USFWS.
FINAL DRAFT Executive Summary i
EXECUTIVE SUMMARY
Introduction: Since the late-1970s, numbers of Double-crested Cormorants (Phalacrocorax
auritus) (DCCO) have increased significantly in many regions of North America. A variety of
problems, both real and perceived, have been associated with these increases, including impacts
to aquaculture, sport and commercial fisheries, natural habitats, and other avian species. Concern
is especially strong over impacts to sport and commercial fishes and aquaculture. Because of
increasing public pressure on U.S. government agencies to reduce DCCO conflicts, the USFWS
is preparing an Environmental Impact Statement (EIS), and in conjunction with the U.S.
Department of Agriculture/Wildlife Services (USDA/WS) and state resource management
agencies, will develop a national management plan for the DCCO. This assessment will be used
to prepare the EIS and management plan.
Populations and trends: The DCCO breeding range in North America is divided into five
geographic areas. Since at least 1980, numbers have clearly increased in three of the breeding
areas: Canadian and U.S. interior, Northeast Atlantic Coast and Southern U.S. In these
populations, much of the growth occurred between the late 1970s – early 1990s; from the early
1990s – 2000 growth rates have slowed or appeared to stabilize in many states and provinces.
For the Pacific Coast and Alaskan breeding populations it was not possible to summarize trends
overall because recent data for birds breeding in significant portions of these regions (e.g.,
Alaska, Mexico) are not available, or have not been collected in a coordinated and timely fashion
for the populations as a whole. Along some parts of the Pacific Coast, breeding numbers
declined in the 1990s (e.g., British Columbia, species is listed as Vulnerable and is being
considered for Threatened status). In other areas significant increases occurred. Concurrently,
numbers also increased on the wintering grounds, particularly in the Mississippi River Delta
region, an area of high human-cormorant conflict over catfish resources.
Many historical records from across the continent indicate that the species was or may have been
more abundant and widespread than is currently presumed. While most of these early accounts
are largely qualitative, many report huge numbers of cormorants, suggesting that recent
population increases may represent recovery towards historical (presettlement) levels in certain
regions. In some areas where the DCCO has been documented as a recent breeder, the species is
actually re-colonizing after an absence of 50 – 300 years.
Reasons for population increases: There appear to be five major factors that led to dramatic
increases in DCCOs in North America since about 1970. These include:
1. Ban on DDT (1972) and other pesticide reduction regulation. Prior to this time (but post
WWII) widespread use of DDT occurred. Cormorants accumulated high levels of DDT
through their food supply, which interfered with reproduction. Depressed populations began
to increase after DDT was banned.
2. In 1972 the DCCO was added to the Migratory Bird Treaty Act protected bird list. Before
1972, federal legislation did not prevent killing or harassment of cormorants during their
annual cycle. Some states also provided special protection for DCCOs around this time.
FINAL DRAFT Executive Summary ii
3. Human induced changes (e.g. accidental and intentional introduction of exotics; over fishing;
changes in water quality) in aquatic communities in the breeding range.
4. Development of aquaculture (e.g. catfish farms) in the south (especially Mississippi Delta
region) that provided a new food source.
5. Creation of additional breeding and foraging habitat (e.g. reservoirs; dredge spoil islands).
Diet and native fish populations: DCCO diet is characterized by great temporal and spatial
variation. The DCCO is known to feed on > 250 species of fresh and saltwater fishes.
Cormorants are generalists and eat abundant fish in the size range 3 – 40 cm; < 15 cm is
preferred. Review of diet studies (> 40) indicates most sport and commercially valuable fish
species do not contribute substantially to DCCO diet. Though there are exceptions, most studies
conclude that sport and commercially valuable fish species are not negatively impacted by
DCCOs, and that DCCOs have minor effects on fish populations relative to human harvest and
other mortality factors. The most common claim against DCCOs is that they reduce sport or
commercial catches, but the actual relationship between cormorant predation, fish population
size and human harvest is poorly understood. This lack of information contributes to the
complexity of cormorant-fish-fishery interactions.
Rigorous quantification of cormorant predation on fish populations or on subsequent sport or
commercial catches requires more precise estimates of several key parameters, including: prey
fish population sizes; prey fish mortality sources and rates; age class distribution of fish
consumed. Additionally, a better understanding of compensatory processes within prey fish
populations is essential (e.g., predation may reduce competition so that remaining fish survive
longer or younger fish grow faster). However, no study conducted so far has obtained robust
estimates for all of these parameters. Therefore, while DCCOs may cause fish populations to
decline, none of the studies reviewed provided data rigorous enough to demonstrate that they do
so. The effect of cormorant predation can be either compensatory (if the cormorants do not eat
them, the same proportion may be removed by other factors) or additive (mortality due to
cormorant predation is not replaced by another factor). However, investigators have rarely
examined cormorant predation in the context of other mortality or limiting factors.
Because of great spatial variation in DCCO diet and unique complexities of individual aquatic
ecosystems, DCCO predation impacts need to be assessed locally. To do this biologists need a
more comprehensive understanding of local fish population dynamics and standardized methods
for assessing cormorant diet.
Diet and aquaculture facilities: Studies show DCCOs may eat large numbers of catfish locally
and temporally. However, no study has quantified the economic impact on net harvest. Only one
study has examined the issue of additive and compensatory mortality and concluded that
mortality due to DCCO predation impacts were additive under certain circumstances, but
insignificant in others.
Impacts on vegetation: Most colonial waterbirds destroy vegetation at breeding and / or
roosting sites to some extent, and cormorants cause some of the most dramatic change.
Cormorants impact vegetation through deposition of guano (excrement) that kills underlying
vegetation and eventually trees, and through nest building behavior when they strip leaves and
FINAL DRAFT Executive Summary iii
small branches. In the short term these changes are of greatest concern if they affect rare plant
communities or private property. From a long-term perspective these changes may be
insignificant on an ecosystem scale. Few studies have been conducted to characterize and
quantify vegetation change due to cormorant nesting and roosting habits.
Impacts on other bird species: DCCOs are hypothesized to have two potential effects on other
colonial waterbird species: competition for nest sites and habitat degradation. Direct interspecific
competition for nests and nest sites may occur but has not been documented through careful
study. Most impacts appear to occur indirectly through habitat degradation (e.g. defoliation, tree
die-off). While there is some evidence that DCCOs may displace other species, no studies have
clearly established DCCO impact on other birds at even a colony level scale.
Management options: Humans have attempted to manage cormorant numbers in the western
hemisphere for at least 400 years. Currently in the U.S. all lethal take requires permits from the
USFWS, except at aquaculture facilities in those states under the 1998 Federal Depredation
Order. Depredation permits can be obtained to prevent economic impacts or impacts to
endangered, threatened or species of conservation concern. Non-lethal harassment of birds
depredating or about to depredate does not require permits. To reduce cormorant impacts
primarily to fisheries, aquaculture, vegetation and other colonial waterbirds, a large number of
techniques has been developed or proposed. These techniques utilize lethal and non-lethal
measures and may be used at local, regional or population levels. The effectiveness of these
measures is difficult to assess because in many cases impacts have been poorly quantified.
Most techniques used at the local level are non-lethal. Lethal control may help reinforce local
non-lethal control techniques. However, because cormorants are highly mobile, lethal control at
the local level may be ineffective at decreasing local populations. Although economic
effectiveness cannot be assessed for individual control techniques, some appear more effective
than others; future research should focus on reducing the costs of the most promising techniques.
Many techniques have been poorly investigated; therefore conclusions about their economic and
numerical effectiveness may be premature. Because aquaculture ponds are high quality foraging
sites (high fish densities; lack of escape cover), control of cormorants on the breeding grounds is
unlikely to eliminate the need to practice local control. To make aquaculture ponds less desirable
foraging sites, some form of control at the local level (e.g. exclosures, harassment) will likely
still be needed. Previous efforts indicate that population control in general must be large scale
and will require sizable human and capital inputs to be effective. Additionally, potential density
dependent effects that compensate for control related mortality are poorly understood.
Addressing these and numerous other uncertainties will enhance the development of a
scientifically based, large-scale population control effort.
Finally, no control is a management option that is economically justified if the costs of control
are greater than the losses associated with cormorant impacts.
Population Models: Models have identified data gaps critical for understanding population
dynamics and predicting control effectiveness; modeling is potentially a very strong tool for
gaining insights into cormorant management. Prediction of future DCCO population trends and
analysis of control methods is hampered by lack of age-specific data for this species. More effort
FINAL DRAFT Executive Summary iv
needs to be put into obtaining data needed to strengthen model predictions, and increased effort
should focus on predicting management outcomes and follow progress. Until better data are
available, however, such modeling efforts should include rigorous sensitivity analyses to
investigate uncertainties in parameters used and assumptions made in the model.
Current research and monitoring efforts: Of 33 U.S. states and nine Canadian provinces to
which surveys were sent, nine reported research in progress and 19 have monitoring programs.
Research addresses: cormorant diet, bioenergetics, impacts to aquaculture, sport and commercial
fisheries, foraging range and foraging behavior. Additional studies are attempting to determine
effectiveness of harassment at day and night roosts, effectiveness of barriers at aquaculture
ponds, and nutrient enrichment in aquatic and terrestrial habitats. A satellite telemetry study will
determine migration patterns, breeding locations and winter movements of cormorants at catfish
farms. All monitoring efforts are used to determine population distribution and trends.
Future research priorities: The assessment identified many research needs. Highest priority
studies on DCCOs fall within the following broad topics: (1) demography, (2) impacts on
fisheries and aquaculture, (3) management techniques, (4) impacts on flora and fauna and (5)
distribution.
FINAL DRAFT List of Figures v
LIST OF FIGURES
Figure 1. Distribution of Double-crested Cormorant (Phalacrocorax auritus) breeding colonies in
North America 1970 – 2000. .............................................................................................15
Figure 2. DCCO Christmas Bird Count sites (1984 – 1998) where ³ 100 birds were counted at
least once. .........................................................................................................................16
Figure 3. December estimates of DCCOs in Alaska based on CBCs, 1984 – 1998.....................18
Figure 4. Distribution and size of active colonies in Alaska at time of last surveys (1970 – 1999).
.........................................................................................................................................19
Figure 5. December estimates of DCCOs on the Pacific Coast, Zone 2*, based on CBCs, 1984 –
1998.................................................................................................................................23
Figure 6. Distribution of Pacific Coast DCCO breeding population (Zone 2), 1970 – 2000. ......25
Figure 7. Distribution and size of active colonies on north Pacific Coast at time of last surveys
(1990s).............................................................................................................................26
Figure 8. Distribution and size of active colonies on south Pacific Coast at time of last surveys
(CA = 1989 – 1998; MX = 1973 – 1992)...........................................................................27
Figure 9. Distribution and size of active colonies in the Pacific Coast southwestern states (1993 –
2000)................................................................................................................................28
Figure 10. December estimates of DCCOs in Arizona based on CBCs, 1984 – 1998. ................30
Figure 11. Changes in number of pairs at Great Chain, Five Finger and Mandarti Islands, Gulf
Islands, B.C., 1987 – 2000.................................................................................................33
Figure 12. December estimates of DCCOs in British Columbia based on CBCs, 1984 – 1998...34
Figure 13. December estimates of DCCOs in California based on CBCs, 1984 – 1998..............38
Figure 14. December estimates of DCCOs at the Salton Sea based on CBCs, 1984 – 1999........38
Figure 15. Changes in number of breeding pairs at Anaho Island, Pyramid Lake, Nevada, 1986 –
1998.................................................................................................................................42
Figure 16. December estimates of DCCOs in New Mexico based on CBCs, 1984 – 1998. ........44
Figure 17. Changes in number of breeding pairs at East Sand and Rice Islands, Columbia River
Estuary, Oregon, 1991 – 2000. ..........................................................................................47
Figure 18. Changes in numbers of breeding pairs at Malheur and Upper Klamath Lakes, 1986 –
1999.................................................................................................................................47
Figure 19. December estimates of DCCOs in Oregon based on CBCs, 1984 – 1998..................48
Figure 20. Changes in breeding numbers at colonies located along the Washington outer coast,
1979-1999.........................................................................................................................50
Figure 21. Changes in breeding numbers at colonies located in theWashington inner coastal
waters, 1983-2000.............................................................................................................51
Figure 22. Changes in breeding numbers in interior Washington, N. Potholes Reservoir, 1978 –
1997.................................................................................................................................51
Figure 23. December estimates of DCCOs in Washington based on CBCs, 1984 – 1998...........52
Figure 24. Changes in numbers of breeding pairs on the Great Lakes, 1970 – 1997...................56
Figure 25. Distribution of Interior U.S. and Canadian DCCO breeding population (Zone 3), 1979
– 2000..............................................................................................................................58
Figure 26. Distribution and size of active colonies in Saskatchewan during last complete survey
(1991). ..............................................................................................................................59
Figure 27. Distribution and size of active colonies on Lake Winnipegosis during last complete
survey (1999). ...................................................................................................................60
FINAL DRAFT List of Figures vi
Figure 28. Distribution and size of active colonies in the Interior west-central states during last
complete surveys (1986 – 1999). .......................................................................................61
Figure 29. Distribution and sizes of active colonies in Interior mid-central states at time of last
complete surveys...............................................................................................................62
Figure 30. Distribution of active colonies (size data not available) in South Dakota and
Minnesota (MN = 1981 – 1995; SD = 1988 – 1992). .........................................................63
Figure 31. Distribution and size of active colonies in Wisconsin during last complete survey
(1997). ..............................................................................................................................64
Figure 32. Distribution and size of active colonies in Michigan during last complete survey
(1997). ..............................................................................................................................65
Figure 33. Distribution and size of active colonies in Ontario during last complete survey (1997).
.........................................................................................................................................66
Figure 34. Detail of Lake Huron showing size and distribution of active colonies (1997). .........67
Figure 35. Distribution and size of active colonies in Ohio, New York, and Vermont................68
Figure 36. Changes in numbers of breeding pairs in Illinois, 1986 – 1997. ................................73
Figure 37. Changes in breeding numbers on Lake Winnipegosis, 1979 – 1999. .........................79
Figure 38. Changes in numbers of breeding pairs in Michigan, 1977 – 1997. ............................81
Figure 39. Changes in number of breeding pairs at Goose Lake, Crescent Lake NWR, Nebraska,
1973 – 1996. .....................................................................................................................86
Figure 40. Changes in number of breeding pairs at Marsh Lakes, Valentine NWR, Nebraska,
1977 – 2000. .....................................................................................................................86
Figure 41. Changes in number of breeding pairs on Little Galloo Island, 1977 – 1998...............88
Figure 42. Changes in numbers of breeding pairs in Ohio, 1992 – 1998. ...................................92
Figure 43. Changes in breeding numbers in Ontario, 1973 – 1997.............................................94
Figure 44. Changes in breeding numbers in Saskatchewan, 1968 – 1991...................................95
Figure 45. Changes in breeding numbers at Bear River Refuge, Utah, 1971 – 1996...................98
Figure 46. Changes in numbers of breeding pairs in Vermont, 1982 – 1999. ...........................100
Figure 47. Changes in breeding numbers in Wisconsin, 1973 – 1997. .....................................102
Figure 48. December estimates of DCCOs in the Southeastern U.S., Zone 4,* based on CBCs,
1984 – 1997. ...................................................................................................................107
Figure 49. Distribution of South and Southeastern U.S. DCCO breeding population (Zone 4)
1980 – 1999. ...................................................................................................................109
Figure 50. Distribution and size of active colonies in South Carolina during last comnplete
survey (1996). .................................................................................................................110
Figure 51. Comparison of Christmas Bird Count sites (with ³ 100 birds) with roost sites located
by USDA/WS in Arkansas, Mississippi and Alabama. ....................................................111
Figure 52. December estimates of DCCOs in Alabama based on CBCs, 1984 – 1997. ............113
Figure 53. December estimates of DCCOs in Arkansas based on CBCs, 1984 – 1997. ............115
Figure 54. December estimates of DCCOs in Florida based on CBCs, 1984 – 1997 ................118
Figure 55. December estimates of DCCOs in Georgia based on CBCs, 1984 – 1997. ..............119
Figure 56. December estimates of DCCOs in Louisiana based on CBCs, 1984 – 1997. ...........123
Figure 57. December estimates of DCCOs in Mississippi based on CBCs, 1984 – 1997..........125
Figure 58. Double-crested Cormorant from Brickell (1737), possibly the first published
representation of this species. ..........................................................................................127
Figure 59. December estimates of DCCOs in North Carolina based on CBCs, 1984 – 1997. ...127
Figure 60. December estimates of DCCOs in Oklahoma based on CBCs, 1984 – 1997. ..........129
FINAL DRAFT List of Figures vii
Figure 61. Changes in number of breeding pairs in South Carolina, 1989 – 1996. ...................131
Figure 62. December estimates of DCCOs in South Carolina based on CBCs, 1984 – 1997. ...132
Figure 63. Changes in number of breeding pairs in Tennessee, 1992 – 1999. ..........................134
Figure 64. December estimates of DCCOs in Tennessee based on CBCs, 1984 – 1997. ..........134
Figure 65. December estimates of DCCOs in Texas based on CBCs, 1984 – 1997. .................137
Figure 66. Changes in numbers of breeding pairs in New England, 1977 – 1994/95. ...............141
Figure 67. December estimates of DCCOs in the Northeast Atlantic, Zone 5*, based on CBCs,
1984 – 1998. ...................................................................................................................142
Figure 68. Distribution of Northeast Atlantic Coast DCCO breeding population (Zone 5), 1980 –
1999...............................................................................................................................145
Figure 69. Distribution and size of active colonies in the mid-Atlantic states at time of last
complete surveys.............................................................................................................146
Figure 70. Distribution and size of active colonies in New England at time of last surveys......147
Figure 71. Distribution and size active colonies in Maine during last complete survey (1994 –
1995)..............................................................................................................................148
Figure 72. Distribution and size of active colonies in New Brunswick, Novia Scotia and Prince
Edward Island during last complete surveys. ...................................................................149
Figure 73. Distribution of active colonies in Quebec at time of last surveys (1970 – 1998)......150
Figure 74. Distribution and size of active colonies in Quebec at time of last surveys (1970 –
1998)..............................................................................................................................151
Figure 75. Detail of the St. Lawrence River Estuary and Gaspe Peninsula showing distribution
and size of active colonies at time of last surveys (1970 – 1998)......................................152
Figure 76. Distribution and size of active colonies along the coast of Newfoundland and on
offshore islands during last complete survey (1996). .......................................................153
Figure 77. Changes in numbers of breeding pairs in Connecticut, 1986 – 1998. ......................155
Figure 78. Changes in numbers of breeding pairs in Maine, 1977 – 1994/95. ..........................158
Figure 79. Changes in number of breeding pairs in Maryland, 1990 – 1999.............................159
Figure 80. Changes in numbers of breeding pairs in Massachusetts, 1977 – 1994/5.................161
Figure 81. December estimates of DCCOs in New Jersey based on CBCs, 1984 – 1998..........166
Figure 82. Changes in breeding numbers in New York (coastal) 1985 – 1998. ........................168
Figure 83. Changes in breeding numbers in Nova Scotia, 1972 – 1992....................................170
Figure 84. Changes in numbers of breeding pairs on Prince Edward Island, 1976 – 1999. .......172
Figure 85. Changes in breeding numbers in Québec, 1970 – 1990...........................................175
Figure 86. Changes in breeding numbers in Rhode Island, 1981 – 1998. .................................177
Figure 87. Annual totals of DCCOs in Virginia based on CBCs, 1984 – 1998. ........................179
Figure 88 ...............................................................................................................................241
Figure 89. The relationship between annual percent increase and r..........................................286
Figure 90. Numbers of Apparently Occupied Nests (AON) on Pigeon Island, Ontario with
population growth estimates. ...........................................................................................286
Figure 91. Annual percent change in number of apparently occupied nests (AON) in Pigeon
Island, Lake Ontario........................................................................................................287
Figure 92. Nesting population change on Gull Island, Lake Superior and logistic model based on
1978 – 1985 nest counts. .................................................................................................294
FINAL DRAFT List of Tables ix
LIST OF TABLES
Table 1. Zone 2, Pacific Coast. Summary of Population data and trends. (+ = increasing; - =
decreasing; 0 = no change). ...............................................................................................24
Table 2. Numbers of chicks fledged at DCCO colonies along the east coast of Vancouver Island
(from Moul 2000)..............................................................................................................33
Table 3. Summary of most recent census data for California breeding regions...........................37
Table 4. Zone 3, Canadian and U.S. Interior. Summary of Population Data and Trends. (+ =
increasing; - = decreasing; 0 = no change).........................................................................57
Table 5. Zone 4, Southern U.S. Summary of Population Data and Trends (+ = increasing; - =
decreasing; 0 = no change). .............................................................................................108
Table 6. Zone 5, Northeast Atlantic Coast. Summary of Population Data and Trends (+ =
increasing; - = decreasing; 0 = no change).......................................................................143
Table 7. Pacific Coast: Occurrence of Salmonids in the diet of Double–crested Cormorants....208
Table 8. Pacific Coast: Occurrence of littoral or estuarine fish in the diet of Double–crested
Cormorants. ....................................................................................................................209
Table 9. Pacific Coast: Occurrence of littoral-benthic fish in the diet of Double–crested
Cormorants. ....................................................................................................................211
Table 10. Pacific Coast: Occurrence of pelagic fish in the diet of Double–crested Cormorants.
.......................................................................................................................................212
Table 11. Great Lakes: Occurrence of open-fresh water fish in the diet of Double–crested
Cormorants. ....................................................................................................................213
Table 12. Great Lakes: Occurrence of littoral-freshwater fish in the diet of Double–crested
Cormorants. ....................................................................................................................215
Table 13. Great Lakes: Occurrence of bottom dwelling-fresh water fish in the diet of Double–
crested Cormorants..........................................................................................................219
Table 14. Great Lakes: Occurrence of Salmonids in the diet of Double–crested Cormorants. ..221
Table 15. Great Lakes: Locally important fish species in the diet of Double–crested Cormorants.
.......................................................................................................................................222
Table 16. Southern U.S.: Occurrence of Shad in the diet of Double–crested Cormorants.........223
Table 17. Southern U.S.: Occurrence of Catfish in the diet of Double–crested Cormorants. ....224
Table 18. Southern U.S.: Occurrence of Centrarchids in the diet of Double–crested Cormorants.
.......................................................................................................................................225
Table 19. Southern U.S.: Locally important fish species in the diet of Double–crested
Cormorants. ....................................................................................................................226
Table 20. Atlantic Region: Occurrence of Marine bottom fish in the diet of Double–crested
Cormorants. ....................................................................................................................226
Table 21. Atlantic Region: Occurrence of Estuarine / Diadromous fishes in the diet of Double–
crested Cormorants..........................................................................................................229
Table 22. Atlantic Region: Occurrence of Freshwater fishes in the diet of Double–crested
Cormorants. ....................................................................................................................232
Table 23. Atlantic Region: Occurrence of Salmonids in the diet of Double–crested Cormorants.
.......................................................................................................................................234
Table 24. Atlantic Region: Occurrence of Pelagic fishes in the diet of Double–crested
Cormorants. ....................................................................................................................235
FINAL DRAFT List of Tables x
Table 25. Atlantic Region: Occurrence of Crustaceans in the diet of Double–crested Cormorants.
.......................................................................................................................................236
Table 26. Sources of information on DCCO control at aquaculture facilities. ..........................243
Table 27. Percent of winter band recoveries (1923-1988) that occurred in the Lower Mississippi
Valley for birds originally banded during the nesting season in various geographic regions
(Dolbeer 1991)................................................................................................................260
Table 28. States and provinces practicing Double-crested Cormorant control. .........................282
Table 29. Impacts in Zone 3 (U.S. and Canadian Interior).......................................................333
Table 30. Impacts in Zone 4 (Southern U.S.)...........................................................................333
Table 31. Impacts in Zone 5 (North Atlantic) ..........................................................................334
Table 32. Summary of fisheries survey data: states / provinces reporting declines linked to
DCCOs. ..........................................................................................................................335
Table 33. Summary of Fisheries Survey Data: Fish species reported to be in decline and factors
thought responsible. ........................................................................................................336
Table 34. States and provinces engaged in current DCCO or DCCO related research or
monitoring programs, and or with identified research priorities. ......................................337
FINAL DRAFT Acknowledgements xi
ACKNOWLEDGEMENTS
Of the hundreds of individuals who contributed to this review, five provided significant
intellectual insight, guidance and friendship: (1) Jeremy Hatch carefully reviewed multiple drafts
of this and related manuscripts, and never failed to provide copious (constructive) criticisms. His
suggestions on organization of the population data were exceptionally helpful and shaped the
entire document. (2) D.V. (Chip) Weseloh, through review of the manuscript and numerous
conversations, provided important ideas on the biology and management of cormorants. His
consistent support, cooperation and encouragement were invaluable. (3) David Carss contributed
in ways too numerous to mention. His publications, review of this assessment, and his
enthusiasm for discussing transatlantic cormorant issues revolutionized our thoughts on
cormorant diet assessment and impacts to fisheries. (4) Thomas Bregnballe provided and helped
us obtain the extensive literature on the Great Cormorant in Europe. He contributed information
that helped broaden our understanding of the complexity of population-level management, and
his work and insights were inspirational. (5) Thomas Keller provided stimulating ideas about
trans-continental cormorant conflicts, particularly on diet, human perceptions and management
issues.
Over fifty people reviewed earlier drafts of this assessment. We are grateful for all of their
comments and suggestions. We especially thank Connie Adams, Robert Ross and Jim Glahn for
providing careful, detailed reviews that challenged our ideas and interpretations, and made us
consider complex issues from multiple perspectives.
Other persons who contributed to our thoughts on cormorants, development of the status
assessment, and problems with piscivorous birds in general include Bill Montevecchi, Bill
Koonz, Walt Lysack and John Trapp. Also, we are grateful to Steve Lewis and Shauna Hanisch
for their support, encouragement and patience; Bruce Vondracek helped organize ecological
groupings for fish species and answered many questions related to fish population dynamics.
Finally, we cannot thank enough the numerous persons identified in Appendix V who
contributed information on biology, distribution, current numbers and recent changes of the
Double-crested Cormorant in North America. Their dedication enabled us to compile an accurate
and detailed status assessment of this species.
The U.S. Fish and Wildlife Service funded preparation of this assessment.
FINAL DRAFT Table of Contents xiii
TABLE OF CONTENTS
EXECUTIVE SUMMARY..........................................................................................................i
LIST OF FIGURES ....................................................................................................................v
LIST OF TABLES..................................................................................................................... ix
ACKNOWLEDGEMENTS ....................................................................................................... xi
TABLE OF CONTENTS......................................................................................................... xiii
INTRODUCTION ......................................................................................................................1
Background Information .........................................................................................................1
The Status Assessment ............................................................................................................2
PART I: NATURAL HISTORY OF THE DOUBLE-CRESTED CORMORANT, Phalacrocorax
auritus .......................................................................................................................................3
GENERAL INFORMATION..................................................................................................3
Taxonomy..........................................................................................................................3
Physical Description............................................................................................................3
Range.................................................................................................................................4
Breeding Range. ..............................................................................................................4
Winter Range. .................................................................................................................4
Habitat ...............................................................................................................................5
Breeding Season Habitat Requirements. ..........................................................................5
Winter Habitat Requirements. .........................................................................................5
BIOLOGY.............................................................................................................................5
Migration ............................................................................................................................5
Reproduction......................................................................................................................6
Foraging Ecology................................................................................................................7
Diet ....................................................................................................................................8
Longevity, Survival and Mortality.......................................................................................8
PART II. POPULATION DATA AND TRENDS.......................................................................9
INTRODUCTION ..................................................................................................................9
Breeding Birds ....................................................................................................................9
Biologically Relevant Units and Organization of Data.....................................................9
Colony Locations ..........................................................................................................10
Trends ...........................................................................................................................11
Wintering Birds.................................................................................................................11
Locations and Winter Distribution.................................................................................11
Use of Christmas Bird Count Data.................................................................................11
Wintering Numbers and Trends .....................................................................................12
Migrant Birds....................................................................................................................12
Migration / Stop-over Sites............................................................................................12
Trends ...........................................................................................................................12
Non-breeding Birds and Total Population Size ..................................................................13
ZONE 1: SUMMARY OF POPULATION DATA FOR ALASKA.......................................17
Introduction......................................................................................................................17
Alaska..............................................................................................................................17
ZONE 2: SUMMARY OF POPULATION DATA FOR THE PACIFIC COAST..................21
FINAL DRAFT Table of Contents xiv
Introduction......................................................................................................................21
Breeding Information ....................................................................................................21
Winter and Migrant Birds..............................................................................................22
Historical Information ...................................................................................................22
Arizona .............................................................................................................................29
British Columbia ...............................................................................................................30
California ..........................................................................................................................34
Mexico.............................................................................................................................39
Nevada.............................................................................................................................41
New Mexico......................................................................................................................42
Oregon.............................................................................................................................44
Washington......................................................................................................................48
ZONE 3: SUMMARY OF POPULATION DATA FOR CANADIAN AND U.S. INTERIOR
POPULATION .....................................................................................................................53
Introduction......................................................................................................................53
Breeding Information ....................................................................................................53
Winter and Migrant Birds..............................................................................................54
Historical Information ...................................................................................................54
Alberta.............................................................................................................................69
Colorado ...........................................................................................................................69
Idaho................................................................................................................................70
Illinois..............................................................................................................................72
Indiana ..............................................................................................................................73
Iowa.................................................................................................................................74
Kansas ..............................................................................................................................76
Manitoba..........................................................................................................................77
Michigan..........................................................................................................................79
Minnesota .........................................................................................................................81
Missouri...........................................................................................................................83
Montana...........................................................................................................................84
Nebraska ...........................................................................................................................85
New York (Inland) ............................................................................................................87
North Dakota.....................................................................................................................89
Ohio .................................................................................................................................90
Ontario ..............................................................................................................................92
Saskatchewan...................................................................................................................94
South Dakota ....................................................................................................................96
Utah .................................................................................................................................97
Vermont...........................................................................................................................99
Wisconsin .......................................................................................................................100
Wyoming ........................................................................................................................102
ZONE 4: SUMMARY OF POPULATION DATA FOR SOUTHEASTERN U.S. ..............105
Introduction....................................................................................................................105
Zone Boundaries. ........................................................................................................105
Breeding Information ..................................................................................................105
Winter and Migrant Birds............................................................................................105
FINAL DRAFT Table of Contents xv
Historical Information .................................................................................................106
Alabama.........................................................................................................................112
Arkansas .........................................................................................................................113
Florida ............................................................................................................................116
Georgia ...........................................................................................................................118
Kentucky........................................................................................................................120
Louisiana ........................................................................................................................121
Mississippi.....................................................................................................................123
North Carolina ................................................................................................................125
Oklahoma.......................................................................................................................128
Puerto Rico / Virgin Islands ............................................................................................130
South Carolina ................................................................................................................130
Tennessee.......................................................................................................................132
Texas ..............................................................................................................................135
ZONE 5: SUMMARY OF POPULATION DATA FOR THE NORTHEAST ATLANTIC
COAST..............................................................................................................................139
Introduction....................................................................................................................139
Breeding Information ..................................................................................................139
Winter and Migrant Birds............................................................................................140
Historical Information .................................................................................................140
Summary of Population Data and Trends.....................................................................154
Historical Information .................................................................................................154
Land Ownership ..........................................................................................................154
Productivity.................................................................................................................154
Delaware........................................................................................................................155
Maine.............................................................................................................................156
Maryland........................................................................................................................158
Massachusetts .................................................................................................................160
New Brunswick...............................................................................................................162
Newfoundland.................................................................................................................163
New Hampshire ..............................................................................................................164
New Jersey......................................................................................................................165
New York (Coastal) ........................................................................................................166
Nova Scotia.....................................................................................................................168
Pennsylvania ...................................................................................................................170
Prince Edward Island ......................................................................................................171
Québec...........................................................................................................................173
Rhode Island ...................................................................................................................176
Virginia..........................................................................................................................177
West Virginia ..................................................................................................................179
PART III: DIET......................................................................................................................181
Introduction ........................................................................................................................181
Diet Studies Reviewed ....................................................................................................181
Diet Assessment Methods: Advantages and Shortcomings ..............................................181
Pellets .........................................................................................................................182
Regurgitations .............................................................................................................183
FINAL DRAFT Table of Contents xvi
Stomach Contents........................................................................................................184
Recovered Tags From Stocked Fish.............................................................................185
Conclusions: Diet Assessment Methods ......................................................................185
Measurements Used to Report Diet .................................................................................185
Results of Diet Review........................................................................................................186
Regional Diet Descriptions..............................................................................................186
Diet on the Pacific Coast .............................................................................................186
Diet in the Great Lakes................................................................................................187
Diet in the Southeastern U.S........................................................................................187
Diet in the Atlantic Region ..........................................................................................188
Conclusions: Variation in the Diet ...............................................................................188
Fish species declines associated with DCCO predation (perceived or real) ......................189
Studies assessing impacts to sport and / or commercial fish populations ..........................190
Great Lakes .................................................................................................................190
Southeastern U.S.........................................................................................................197
Conclusions: Impacts to Sport and / or Commercial Fisheries......................................199
Studies assessing impacts to aquaculture .........................................................................200
Surveys .......................................................................................................................201
Diet Studies and Observations at Ponds.......................................................................202
Bioenergetics Modeling...............................................................................................205
Conclusions: Impacts to Aquaculture...........................................................................207
Studies on food preferences.............................................................................................207
PART IV. IMPACTS TO VEGETATION AND COLONIAL WATERBIRDS ......................237
Impacts to Vegetation .........................................................................................................237
Impacts to avian species ......................................................................................................238
Newcastle Disease ..............................................................................................................239
PART V. MANAGEMENT OPTIONS...................................................................................241
Introduction ........................................................................................................................241
Managing Impacts to Aquaculture.......................................................................................244
1) Exclusion Devices: Physical and Functional Barriers ..................................................244
Netting ........................................................................................................................244
Wires..........................................................................................................................245
Floating Ropes ............................................................................................................246
Other Flight Inhibitors.................................................................................................247
Underwater Exclusion Devices....................................................................................247
2) Non-lethal Harassment at Aquaculture Facilities .........................................................247
Human Harassment .....................................................................................................247
Simulated Human Harassment.....................................................................................249
Other Harassment ........................................................................................................250
3) Lethal Control at Aquaculture Facilities ......................................................................251
Reinforcing Non-lethal Harassment Techniques ..........................................................251
Reducing Local Populations ........................................................................................252
Accidental Killing of Other Species.............................................................................253
4) Non-lethal Harassment at Night Roosts.......................................................................254
5) Provision of Alternative Prey and Foraging Sites ........................................................255
Provision of Alternative Prey.......................................................................................255
FINAL DRAFT Table of Contents xvii
Alternative Foraging Sites ...........................................................................................256
6) Alteration of Aquaculture Practices.............................................................................256
Decrease Fish Densities...............................................................................................257
Pond Dyes...................................................................................................................257
Water Turbulence........................................................................................................257
Aquaculture Facility Location .....................................................................................258
Improved Facility Design ............................................................................................258
Alter Stocking Practices ..............................................................................................258
Passive Harassment .....................................................................................................259
Alteration of Fish Behavior .........................................................................................259
Chemical Avian Repellants .........................................................................................259
Conditioned Taste Aversion ........................................................................................259
Manage Other Avian Species.......................................................................................259
7) Population Control......................................................................................................259
8) No Control..................................................................................................................262
9) Conclusions: Managing Impacts to Aquaculture..........................................................262
Managing Impacts to Vegetation and Habitat ......................................................................263
Techniques.....................................................................................................................264
Harassment of Adults ..................................................................................................264
Nest / Tree Removal....................................................................................................265
Egg and Nestling Destruction ......................................................................................266
Egg Oiling / Spraying..................................................................................................268
Avian Contraceptives ..................................................................................................270
Killing Adults..............................................................................................................270
No Control ..................................................................................................................272
Case Studies: Multiple Techniques..................................................................................272
Lake Winnipegosis, Manitoba .....................................................................................273
St. Lawrence Estuary, Québec .....................................................................................274
Lake Champlain, Vermont (USFWS 1999a)................................................................275
Lake Ontario, New York (USFWS 1999b) ..................................................................277
Conclusions: Managing Impacts to Vegetation and Habitat .............................................278
Managing Impacts To Fisheries...........................................................................................278
Hatchery-raised Trout Releases .......................................................................................278
Salmon Smolt Releases / Runs ........................................................................................279
Nets / Weirs ....................................................................................................................279
Equipment Modification..............................................................................................280
Harassment..................................................................................................................280
Alternative Perches and Prey.......................................................................................281
Reduce Time Fish Spend in Nets / Weirs.....................................................................281
4) Conclusions: Managing Impacts To Fisheries..............................................................281
Survey Results ....................................................................................................................281
PART VI. POPULATION MODELING.................................................................................283
INTRODUCTION ..............................................................................................................283
DETERMINISTIC EXPONENTIAL MODELS .................................................................284
Basic Exponential Growth Models ..................................................................................284
Age-Structured Exponential Growth Models ...................................................................288
FINAL DRAFT Table of Contents xviii
Lake Ontario ...............................................................................................................288
St. Lawrence Estuary, Québec .....................................................................................289
Lake Champlain, Vermont...........................................................................................291
DETERMINISTIC LOGISTIC MODELS...........................................................................293
Basic Logistic Growth Models ........................................................................................293
Age-Structured Logistic Growth Models .........................................................................294
Great Cormorants in Europe................................................................................................295
CONCLUSIONS ................................................................................................................296
PART VII. RESEARCH NEEDS............................................................................................297
DISTRIBUTION INFORMATION.....................................................................................297
DEMOGRAPHIC INFORMATION ...................................................................................297
DIET..................................................................................................................................298
IMPACTS TO FISHERIES AND AQUACULTURE..........................................................298
IMPACTS TO FLORA AND FAUNA................................................................................299
MANAGEMENT TECHNIQUES.......................................................................................299
ECONOMIC IMPACTS .....................................................................................................300
LITERATURE CITED...........................................................................................................301
APPENDIX I. PERCEIVED IMPACTS ASSOCIATED WITH DOUBLE-CRESTED
CORMORANTS.....................................................................................................................325
Survey Response.................................................................................................................325
Impacts to commercial / sport fish (reported by wildlife biologists, etc.)..........................325
Impacts to commercial / sport fish (reported by fisheries biologists) ................................326
Impacts to Aquaculture ...................................................................................................327
Impacts to Vegetation......................................................................................................328
Impacts to avian species ..................................................................................................329
Impacts to other fauna .....................................................................................................330
Damage to Private Property.............................................................................................330
Disease Transmission......................................................................................................331
Impacts to Water Quality.................................................................................................331
Impacts to Public Health .................................................................................................332
APPENDIX II. CURRENT RESEARCH................................................................................337
APPENDIX III. LIST OF COMMON AND SCIENTIFIC NAMES .......................................343
Birds ..................................................................................................................................343
Fishes.................................................................................................................................344
Mammals ............................................................................................................................346
Arthropods.........................................................................................................................347
Other Invertebrates..............................................................................................................347
Plants.................................................................................................................................347
APPENDIX IV. LIST OF ABBREVIATIONS .......................................................................349
APPENDIX V. LIST OF PERSONAL COMMUNICATIONS...............................................351
FINAL DRAFT Introduction 1
INTRODUCTION
Background Information
Since the 1970s, numbers of Double-crested Cormorants (Phalacrocorax auritus)
(DCCO) have increased significantly in many regions of North America. A variety of problems,
both real and perceived, have been associated with these increases, including impacts to
aquaculture, sport and commercial fisheries, natural habitats, and other avian species. Concern is
greatest over impacts to sport and commercial fishes, and to aquaculture. Impacts to the latter
appear more significant, but studies thus far have not determined the magnitude of these impacts.
The DCCO is protected in the United States by the Migratory Bird Treaty Act (MBTA)
of 1918, as amended (16 U.S.C. 703-712). DCCOs were added to the list of protected species (50
Stat. 1311, T.S. No. 12) in 1972 following amendment of the U.S. Convention with Mexico (23
U.S.T. 260, T.I.A.S. 7302). DCCOs are not included in the U.S. Convention with Great Britain,
acting on behalf of Canada for the Protection of Migratory Birds (39 Stat. 1702 T.S. No. 628 ),
and are therefore not protected by the Canadian Federal Government, although they receive
protection by provincial governments. In addition to U.S. federal protection, DCCOs may also
receive some protection at the state level.
Due to various concerns, legal actions have been taken to control DCCO numbers in the
U.S. In March, 1998, USFWS established a depredation order (Trapp 1998) in response to
complaints from catfish farmers and baitfish dealers. This order allows those engaged in
commercial aquaculture to shoot cormorants without a federal permit at freshwater aquaculture
premises or state-operated hatcheries in Minnesota and 12 southeastern states. Cormorant control
programs also exist in individual states to control numbers to reduce their impacts to island
vegetation and other colonial waterbirds, but these can only be carried out under the terms of a
federal permit.
Illegal actions to control cormorants have also occurred. In the summer of 1998,
frustrated fishermen who believed cormorants were responsible for game fish declines in the
eastern basin of Lake Ontario illegally shot an estimated 1500 – 2000 Double-crested
Cormorants on Little Galloo Island, Lake Ontario. In 1999, the State of New York requested a
depredation permit to reduce cormorants on eastern Lake Ontario primarily for the purpose of
controlling their predation on smallmouth bass, a popular sport fish. The State’s secondary
concern was to limit cormorant competition with other bird species. The USFWS issued a permit
to prevent all reproduction through egg oiling on Little Galloo Island, and to continue nest
destruction efforts on other islands on Lake Ontario to benefit other bird species and their
habitats. In Canada, legal and illegal control for similar reasons has also occurred.
A resolution was signed by George W. Bush, then the governor of Texas, asking USFWS
to evaluate the economic impact of the DCCO on sport fishing and to “consider removal of the
Double-crested Cormorant from the protection of the Migratory Bird Treaty Act if national
economic losses warrant severe control methods to keep the cormorant population at a
manageable level within a given region.” Because of these strong concerns over potential
impacts associated with increases in DCCOs, the USFWS is preparing an Environmental Impact
Statement (EIS), and in conjunction with the U.S. Department of Agriculture / Wildlife Services
(USDA/WS) and state resource management agencies, plan to develop a national management
plan for the DCCO. This status assessment will serve as a primary resource to aid in the EIS and
development of the management plan.
FINAL DRAFT Introduction 2
The Status Assessment
To assess the status of the Double-crested Cormorant, we reviewed natural history,
population data and trends, diet studies, cormorant impacts to vegetation and colonial waterbirds,
cormorant control, population modeling, current research needs and future research priorities.
Much of this information was obtained through a review of published literature, literature in
press, and unpublished reports. In addition to summarizing information, we evaluated some
studies (mainly those which attempted to determine impacts of DCCO predation) for accuracy
and thoroughness. Similar increases in numbers of the Great Cormorant (Phalacrocorax carbo)
(GRCO) have occurred in Europe, and similar conflicts with human interests have arisen.
Because the DCCO and the European GRCO are ecological counterparts, and much significant
research has been conducted on the latter species, we incorporated relevant insights gained
through study of the GRCO.
We also conducted two surveys to obtain information. We first conducted a DCCO
survey to obtain data on breeding, wintering and migration numbers and distributions, population
trends, and current research, monitoring and management / research priorities; and to obtain
information on the perceptions of impacts to natural resources and damage problems associated
with DCCOs in each state / province. We sent this survey to agency (both government and non-government)
wildlife biologists, university and museum biologists, and to USDA/WS personnel
in areas where problems associated with cormorants are high. Survey recipients were asked to
identify a fisheries biologist to whom we could address specific questions regarding impacts to
sport and commercial fish. We then conducted a second survey with fisheries biologists to obtain
more information on perceptions of impacts to fish and to gain a wider perspective on this
problem. Where questions arose regarding information provided in the surveys, we made follow-up
calls and tried to resolve unclear issues.
FINAL DRAFT Part I. Natural History 3
PART I: NATURAL HISTORY OF THE DOUBLE-CRESTED
CORMORANT, PHALACROCORAX AURITUS
[This section is a brief summary based largely on the Birds of North America species
account of the Double-crested Cormorant (Hatch and Weseloh 1999). Parts of some sections
(breeding and winter range) come directly from this account; for a full treatment of the natural
history of this species and for specific data sources the reader should see account. Other
literature and information sources were also occasionally included.]
GENERAL INFORMATION
Taxonomy
The Double-crested Cormorant (DCCO) is in the order Pelecaniformes, family
Phalacrocoracidae. Worldwide, between 30 – 40 species of cormorants are recognized. In earlier
taxonomies most species were placed in the single genus Phalacrocorax; recent accounts favor
dividing the family into two main groups, the typical cormorants and the strictly maritime ones,
the “shags.” In North America, there are six species of cormorants, including the Double-crested;
these include Great Cormorant (P. carbo), Neotropic Cormorant (P. brasilianus) [formerly called
Olivaceous Cormorant (P. olivaceus)], Brandt’s Cormorant (P. penicillatus), Pelagic Cormorant
(P. pelagicus), and Red-faced Cormorant (P. urile). The range of the Double-crested overlaps
those of Brandt’s and Pelagic Cormorants on the Pacific Coast, from southern Alaska to the Baja
Peninsula; the Red-faced Cormorant mostly along the coast of southern Alaska; the Neotropic in
Texas, Louisiana, and w. Mexico, mainly Sinaloa; and the Great Cormorant along the east coast,
from Newfoundland to Florida. The Double-crested is most closely related to the Neotropic
Cormorant and is in the same subfamily (genus) as the Great (carbo) and Brandt’s (penicillatus)
cormorants. The other subfamily includes species provisionally named as shags.
Five subspecies of the Double-crested Cormorant have been described, based on size and
crest characters: 1) P.a. auritus (Northern Double-crested Cormorant), most numerous, breeds
widely in the interior and on the northeast coast; 2) P.a. cincinatus (formerly White-crested or
White-tufted Cormorant) occurs in Alaska; 3) P.a. albociliatus (formerly Farallon Cormorant)
breeds on Pacific Coast and inland, possibly to New Mexico, Utah and Montana; 4) P.a.
floridanus (formerly Florida Cormorant) is resident in Florida and Caribbean; 5) P.a. heuretus
(no common name) is resident on San Salvador I., and possibly other islands of the Bahamas (see
figures in Palmer 1962, p. 331; Johnsgard 1993, Fig. 51, p. 200). Characteristics of birds
breeding in Mexico have not been established. Banding recoveries suggest that little mixing
occurs across the Rocky Mountains.
Physical Description
The DCCO is a medium to large (typical length 70 – 90 cm, body mass 1.2 – 2.5 kg) dark
waterbird. Cormorants have elongated bodies, moderately long necks and bills, long wedge-shaped
tails, and totipalmate feet. Resting birds often hold wings in a spread-wing posture,
thought to aid in drying wet feathers. Distinctive characteristics of the DCCO include brilliant
turquoise eyes, a golden throat pouch, elegantly patterned back feathers, and for a short time
prior to breeding season, a cobalt-blue mouth. Plumage is black or dark brown with a dull
FINAL DRAFT Part I. Natural History 4
greenish or bronze gloss. There is considerable variation in body size, and in color and size of
crest. Colors of eye-lid may vary geographically or individually from blue to orange. The genus
name, Phalacrocorax, is Greek for “bald-headed raven”; the species name, auritus, is Latin for
“eared” and refers to the crests above the eyes, feathers which are fully developed only for a
short time early in the breeding season.
Range
Widely distributed in North America, but not elsewhere. Vagrants have been reported
from England 1989, and the Azores in 1991.
Breeding Range.
Five major breeding zones have been defined:
1) Alaska
2) Pacific Coast
3) Canadian and U.S. Interior
4) Atlantic Coast
5) Florida and the western Caribbean
The five breeding zones are partly reflected in subspecific designations. Populations of
P.a. auritus have expanded and contracted from two areas: freshwater breeders in the Prairie
provinces, and maritime birds in the northwest Atlantic. The other four subspecies are
concentrated in Alaska (P.a. cincinatus); along the Pacific Coast (P.a. albociliatus); in Florida
and the Caribbean (P.a. floridanus); and in the Bahamas and Cuba (P.a. heuretus). The Alaskan
population breeds at Nunivak Island., and se. Bering Sea from e. Aleutian Islands. to the
southeast coast, including Kodiak Island. The Pacific Coast population breeds between s. British
Columbia and Sinaloa, Mexico; most breeding is coastal, though some occurs inland. The
Canadian and U.S. interior population breeds from n. Alberta through central Ontario; James Bay
and sw. Québec, south to central Utah, central Colorado, w. central Nebraska, se. South Dakota,
w. central Minnesota, ne. Iowa, central Wisconsin, and n. Lower Peninsula of Michigan; range
extends west to sw. Idaho and east along GL to lower St. Lawrence River. Also breeds locally in
other areas within the interior (see Hatch and Weseloh 1999). On the Atlantic Coast, breeds from
Newfoundland to New York, and small numbers elsewhere (see Hatch and Weseloh 1999). Most
breeding is coastal. In the southeast, resident in Florida, and occurring locally in small numbers
along Gulf Coast to Texas. Scattered residents in Cuba, and uncommon resident in Bahamas.
Also nests on coast of Yucatan Peninsula and n. Belize. Recent expansion has led to blurred
boundaries for Zones 3, 4 and 5. For further discussion and delineation of the five breeding
zones, see Part II, Biologically Relevant Units and Organization of Data.
Winter Range.
On the Pacific Coast and Alaska, chiefly resident, though some dispersal occurs. Birds
breeding in the interior and on the Atlantic Coast are strongly migratory, and most winter
coastally from North Carolina to the western Gulf of Mexico. Also winters along the Gulf of
Mexico from Tamaulipas south to the Yucatan Peninsula and Belize. Inland, significant numbers
occur at lakes, rivers and impoundments; regularly winters inland from Atlantic Coast along
major rivers to se. Pennsylvania, central Maryland, and throughout lower coastal plains of
FINAL DRAFT Part I. Natural History 5
Virginia and the Carolinas. Also winters inland from Gulf Coast (especially along the
Mississippi and other rivers) north to central Georgia, n. Tennessee, sw. Indiana, s. Illinois, n.
Arkansas, e. Kansas, eastern half of Oklahoma, Texas (except for Panhandle). Small numbers are
increasingly observed in coastal New England, the Maritimes, the Great Lakes, and the prairies.
Other populations breeding in Florida, Mexico, Cuba, and the Bahamas are resident.
Habitat
Breeding Season Habitat Requirements.
Colonies require sites safe from ground predators and close to feeding areas (usually < 10
km). Ponds, lakes, slow-moving rivers, lagoons, estuaries and open coastlines are utilized. Where
available, selects small rocky or sandy islands. May also use artificial sites such as bridges,
wrecks, abandoned docks or purpose-built towers. Though nests on ground or in trees, ground
nesting may be the ancestral and preferred habit; tree nesting may be response to predators. Trees
used for nesting are usually standing in or near water, on islands, in swamps, or at tree-lined
lakes. Where predators are present, depends on flooded snags or live riparian trees. Also nests on
emergent vegetation in marshes. In all seasons requires suitable places for nighttime roosts and
daytime resting or loafing. Roosts and resting places are often on exposed sites such as rocks or
sandbars, pilings, wrecks, high-tension wires or trees near favored fishing sites.
Winter Habitat Requirements.
Largest numbers of wintering birds occur along the southern coasts, and require similar
characteristics in feeding, loafing and roosting sites as when breeding. Cormorants wintering
along the North Carolina coast roost in or near the inlets on sandbars in dense flocks of up to
5,000 – 10,000 birds, or more, so that the sandbars are often blackened. They roost so close they
are nearly touching each other. Sandbars or shoals chosen are high enough that they are not
underwater at high tides. During the day, birds feed in large flocks, often numbering several
thousands, in the inlet, sounds, and inshore ocean. Very little roosting is in trees, but some roost
on channel markers, pilings, etc. Birds also rest on sandbars and markers during the day. They
are often seen flying to sandbars, and then flying out to sea in the afternoon; at dusk they return
to the sandbars (H. LeGrand, pers. comm.). Around aquaculture sites winter numbers are
increasing, and in Mississippi many that winter near catfish farms roost in isolated cypress
swamps. There may be differential selection between sexes in winter habitat use; in Mississippi,
males predominated in samples shot at inland roosts (Glahn et al. 1995), and in Texas females
predominated in a sample collected in Houston Ship Channel (King et al. 1987).
BIOLOGY
Migration
Atlantic Coast and interior nesting birds are highly migratory; all age groups migrate. In
other areas, the species is mostly resident within the breeding range and migratory habits are
poorly understood. More northerly populations breeding in Alaska may migrate south to s.
British Columbia and Washington.
Migrants wintering in the Gulf of Mexico begin heading northward as early as February.
Along the eastern seaboard they follow the Atlantic coastline; in the interior they follow river
systems and fly overland. Earliest individuals reach Oklahoma and Virginia by 4 – 5 March;
FINAL DRAFT Part I. Natural History 6
Massachusetts, the southern Great Lakes, Minnesota, S. Dakota, and s. Idaho by late Mar – early
April; upper Great Lakes by early to mid April; more northerly destinations, e.g., the prairie
provinces, coastal Maine, are not reached until mid-late April. In British Columbia, migration
occurs mid-Mar to late May, peaking late April – early May. Analysis of banding recoveries
indicates that first-year birds return north about a month later than older birds. Recently, some
birds have been remaining year-round and breeding along the Gulf Coast.
Autumn migration is essentially the reverse of spring. Earliest autumn migrants appear to
reach wintering grounds faster than they reached breeding grounds in spring. Migration
underway on the Canadian prairies by mid-late August, at which time first migrants are arriving
on the Upper Mississippi River, Ohio, Massachusetts, Virginia and Texas. By September,
migration well underway throughout range. Massachusetts, Rhode Island and Minnesota report
peak numbers of transients in October. Most banded migrants recovered in Oct and Nov. Along
coastal New England, bulk of migration observed 25 September – 17 October. Individuals arrive
on winter grounds September to November. Some fly across open ocean.
Inland, migrates in small flocks, < 50 – 100; along coasts, near shore flocks may consist
of 1000s of birds. Migratory flight begins soon after dawn and continues all day, with flocks
observed flying late in evening.
Reproduction
Usually forms dense breeding colonies (100s – 1000s); solitary nests very rare. Often
nests in mixed-species colonies. DCCOs are believed to have extreme fidelity to colony sites.
Many young first breed at natal colonies, and philopatry to proximity of natal colony is probable;
in banded birds recovered in June that were at least 3 years old, the median distance to natal
colony was only 25 km.
In British Columbia, most DCCOs began breeding as 3 year olds (in their 4th summer).
However, a small portion of color banded birds first bred as 1 year olds (4.7 %) and 2 year olds
(16.5 %). Immatures (1 and 2 yr olds) are present on breeding colonies by June.
Ground nesting cormorants typically nest on low-lying rock islands or reefs, away from
or with sparse vegetation. Arboreal nesters use evergreen and deciduous trees, initially alive but
killed within 3 – 10 years due to guano deposition over time. Nesting substrates include
limestone and granite bedrock, large boulders or fallen trees, gravel, cobbles, beach ridges, soil
or standing trees. Possibly usurps attended or unoccupied nests of some herons and egrets. Nest
characteristically includes finger-sized sticks and other bulky items collected from diverse
locations, some several kilometers away. Along seacoasts seaweed forms the usual nest-base;
flotsam and jetsam often incorporated, as are parts of dead birds. Lining is usually of grass,
rootlets and similar materials. As breeding season progresses, the nest receives pebbles and
bones from pellets, and a coating of guano on the outside that seals the nest together. Many nests
are used repeatedly and built upon each year, becoming tall turret-like structures; some reach
heights of 2 m or higher. Tree nests are usually much smaller but have a deeper inner cavity than
ground nests. Ground nesters defend small area around nest within beak range (0.38 m).
Reported inter-nest distances vary widely, from 3.3 – 220 cm.
Double-crested cormorants lay between 1 – 7 eggs. Average clutch size ranges from 2.7 –
4.1; modal clutch size is four. First eggs are laid 2 – 4 weeks after arrival to breeding colonies. In
Ontario, egg laying occurs late Apr to Aug; in the St. Lawrence River estuary from about 10
May to Jul. Eggs are cylindrical ovate in shape. Shells are pale blue and unmarked, but
pigmented layer often obscured by variable outer calcite cover that is initially white and porous.
FINAL DRAFT Part I. Natural History 7
Calcite layer has chalky texture and gives irregular surface. Egg mass is small compared to other
seabirds; reported averages range from 44.9 – 46.5 g, 2.7 % of adult mass. Eggs are laid daily or
on alternate days. Following loss of complete clutch, relaying is frequent, occurring within 19
days. However, successful second broods are rare. Eggs are extremely cold tolerant, but
vulnerable to heat stress.
Incubation begins gradually, reaching a maximum intensity when third egg is laid. Eggs
are incubated by lying on top of warm webs of feet; abdomen and breast are lowered onto them.
Duration of incubation varies from 25 – 28 days. Both sexes incubate, but the female does more
during the first half of the incubation period. Changeovers occur at intervals of 1 – 3 hours.
Hatching occurs asynchronously, but intervals between eggs typically less than 1 day.
Chicks are altricial, and barely able to move at hatching. Eyes open at 3 – 4 days; egg
tooth drops off between 4 – 7 days. Down appears within about one week. Thermoregulatory
ability not complete until 2 weeks. Young are cared for equally by both parents. Brooding is
nearly continuous for the first 12 days, but ceases with the appearance of down and effective
endothermy. Young will remain in ground nests for about 28 days if undisturbed (though have
walked from ground nests as early as 21 days when approached by humans or predators). Young
in tree nests or in nests on cliffs may remain there until they are able to fly, which occurs
between 6 – 8 weeks of age. Diving ability develops and plumage is complete at about the same
time. Young are completely independent of parents at 10 weeks of age.
Annual reproductive success varies within and among colonies, but hatching success
usually 50 – 75 %; fledging success 1.2 – 2.4 young / nest or 74 – 95 %. Chick loss from
hatching to fledging is often low, e.g. 5 % in coastal B.C. All figures much lower for DDE-contaminated
populations. In St. Lawrence estuary, reproductive success parameters lower for
late nesting cormorants (June) compared to early nesters (May). Average lifetime production
(lifetime reproductive success) calculated for birds breeding on Mandarte I., B.C. was 3.28
young (van der Veen 1973).
Foraging Ecology
The DCCO is an opportunistic, generalist feeder, preying mainly upon abundant, easy-to-catch
fish species. Usually slow-moving or schooling fish, ranging in size from 3 – 40 cm but
commonly < 15 cm, are taken. The DCCO appears to be strictly diurnal in its feeding habits, and
usually forages in shallow water (< 8 m) within 5 km of the shore. The prey of Atlantic birds
suggests that they are more likely to feed at the bottom of the water column, while that of Pacific
and inland birds suggests that they feed in mid-water. Cormorants respond rapidly to high
concentrations of fish and often congregate where fish are easily caught, such as put and take
lakes, stocking release sites, aquaculture ponds, dams, and other areas where fish are
concentrated.
To capture food, cormorants dive from the surface and pursue prey underwater. Prey is
grasped in the bill and may be swallowed underwater. If the prey is large or difficult to handle,
such as eels, flounders or spiny fish, it may be swallowed at the surface after first being shaken
and hammered on the water. Prey is sometimes thrown in the air, caught and swallowed head-first.
Frequently forages individually, but readily gathers to form feeding flocks (tens to
hundreds). When feeding on schooling prey, sometimes loosely-coordinated foraging flocks are
formed. This behavior has been observed most often in the breeding season and in late summer /
early fall. Bottom-feeding is usually solitary.
FINAL DRAFT Part I. Natural History 8
Diet
Information on the diverse diet of this species has been recorded from as early as 1835,
when Audubon (1835) observed that the food of Double-crested Cormorants consisted of
“shrimps, lents, capelings, codlings, and other fishes”. In general, primary prey are forage
species of little or no commercial value, but cormorants will take advantage of abundant species
in the right size range. Over 250 fish species from more than 60 families have been reported as
prey items. Occasionally, other aquatic animals, such as insects, crustaceans and amphibians are
also taken. Diet is discussed specifically by region in PART III: DIET.
Longevity, Survival and Mortality
Oldest banded bird reported was 17 years, 9 months of age; however, wear and loss of
aluminum bands is likely to lead to underestimates of survival based on recoveries. For birds
banded as fledglings on Mandarte I., B.C., first year survival was estimated at 0.48, second-year
at 0.74, and subsequent annual survival of 0.85; mean adult life expectancy 6.1 year (van der
Veen 1973). Mortality factors for young birds include disturbance at breeding colonies, which
can result in large mortality of hatchlings from exposure, and of eggs and young by predation;
Newcastle disease can also cause significant mortality among young birds. Adult and large chick
mortality factors include predation by Bald Eagles (Haliaeetus leucocephalus), entanglement in
fishing gear, and shooting. Rates of mortality due to predation were not available. Fishing gear is
reported as a major cause of death. Of birds banded in the Great Lakes between 1928 – 1995, 9
% of those recovered were shot; of band recoveries in Texas, 17 % were shot.
FINAL DRAFT Part II. Population Data and Trends, Introduction 9
PART II. POPULATION DATA AND TRENDS
INTRODUCTION
Most population and trend data were obtained through the survey sent to wildlife
biologists. We also utilized published literature, breeding bird atlases, unpublished reports and /
or papers in press regarding trends and / or the history of cormorants in specific states and
provinces. Survey recipients were asked to provide data on breeding, wintering, and migration
concentrations. For breeding birds we asked recipients if cormorants were recent (1972 or later)
breeders in their state / province, and to provide number of pairs, ownership of site (if colony >
300 pairs), and any productivity estimates. For wintering populations, we asked for number of
birds and site ownership information (if concentrations > 1000 birds). For migration
concentrations, we asked for peak migration times and numbers and land ownership for
significant stopover sites (concentrations of > 500 birds). We also telephoned and e-mailed many
individuals who provided us with data to get more precise information regarding numbers and
trends in their states and provinces. Individuals completing surveys sometimes provided possible
reasons for population increases; these were incorporated into the state and provincial profiles.
Breeding Birds
Biologically Relevant Units and Organization of Data
Five main zones for breeding Double-crested Cormorants in North America were defined
by Hatch and Weseloh (1999) and these largely correspond to subspecies ranges (see Part I:
Breeding Range.). In an effort to present biologically relevant information on biological units, or
“populations”, we organized and presented population data based largely on these population
zones, with a few modifications. In North America, populations of breeding cormorants occupy
five main areas: Alaska (Zone 1); the Pacific Coast, from British Columbia to Mexico (Zone 2);
the Canadian and U.S. interior (Zone 3); the Southeastern U.S (Zone 4), and the Atlantic Coast,
from Newfoundland to New York (Zone 5) (Figure 1). For the most part, these zones are
geographically or biologically (in terms of subspecies) distinct. Data were organized in this
manner to aid and encourage individuals to view the biological unit, rather than political or
organizational boundaries, when developing management plans.
These zones are thought to reflect fairly distinct breeding populations, but recent DCCO
expansion and re-colonization has blurred the boundaries between these zones (Hatch and
Weseloh 1999). Border states between interior and southeastern populations, interior and western
populations, and southeastern and northeast Atlantic populations (e.g., Texas, New Mexico,
Idaho, inland southern states, the Carolinas), qualify as “gray areas.” In these areas, it is not
possible to determine precisely which zone breeding cormorants belong to; therefore, it was not
possible to delineate absolute boundaries for each zone. However, because we were interested in
presenting as much biologically relevant information as possible, and birds breeding in “gray
areas” were in relatively small numbers, we determined the most likely population zone for each
colony documented between 1970-2000 (Figure 1). These determinations were based on the best
information available on subspecies range (Palmer 1962; Johnsgard 1993; Hatch and Weseloh
1999), and consideration of logical geographic units.
Additionally, to help with decisions for birds breeding in inland southern states (AR, OK,
TN), southeastern coastal states (NC, SC), and southwestern states (TX, NM), we also
FINAL DRAFT Part II. Population Data and Trends, Introduction 10
considered winter distribution, and the fact that birds breeding in many of the southern states are
likely year-round residents, while birds from neighboring Zones 3 and 5 are migratory. The
winter range in North America is quite distinct and occupies two main areas: in the eastern and
central portion of the continent, the species winters mostly from Texas to Florida along the Gulf
Coast, and along the Atlantic Coast through the Carolinas and Virginia; in western North
America, the species winters mostly along the Pacific Coast, from southern British Columbia to
Baja California Sur and the Gulf of California. Large numbers also winter inland in Texas,
Oklahoma, and Arkansas. The large number of birds wintering in the eastern and central portion
is a fairly easily defined, succinct, and very important unit in terms of human-cormorant
conflicts. Therefore, it made the most sense to keep the southern states together and acknowledge
the possibility that the small number of birds that breed in the boundary states of this zone may
in fact be part of the interior or northeast Atlantic populations. For birds breeding in Idaho,
where some have been found to possess characters intermediate between subspecies P.a.
albociliatus and P.a. auritus (Burleigh 1972), suggesting that mixing occurs at the “boundaries”
of Zones 3 and 5, we decided to rely on Burleigh’s (1972) conclusion that Idaho birds examined
came closer to P.a. auritus than to P.a. albociliatus; thus we grouped Idaho birds with the
interior population.
Therefore, when viewing these population zones, we suggest that special attention be
paid to boundaries, especially “gray areas”. Extent and areas of overlap need to be more closely
defined, especially if management actions are considered at a regional level or potential impacts
to a population are under review. More detail is presented on population organization in the
individual summaries for the zones; the problem is also addressed in PART VII. RESEARCH
NEEDS.
Colony Locations
Survey recipients were asked to provide latitude-longitude coordinates for each colony
location known since 1970. If these data were not available, we made follow-up calls and tried to
determine approximate locations. We were able to obtain relatively good location data for most
colonies and have included maps showing the distribution of colonies in each population zone
during the last 30 years (Figure 1). In Zone 1 we show distribution of all known colony sites. In
Zone 2 we show distribution of all known colony sites with the exception of interior California.
D Shuford at Point Reyes Bird Observatory will supply a map for this area (D. Shuford, pers.
comm.). In Zone 3, we were unable to obtain complete data for the following states and
provinces: for Iowa, we were unable to obtain coordinates for 4 of 10 known colonies; for
Minnesota, we were unable to obtain coordinates for 7 of 75 known colonies; for Alberta,
Montana, North Dakota and Colorado we were unable to obtain any precise information. In Zone
4, we obtained coordinates for all known breeding colonies, with the exception of Florida, where
coordinates were provided for “most” cormorant colonies in the state. In Zone 5, we obtained
coordinates for all known colonies except for 5 of 12 on Prince Edward Island, and 2 of 9 in
Newfoundland.
Maps showing distribution of active colonies by size at time of last survey are included
for states and provinces for which we have the appropriate data in the five breeding zones. In
Zone 4, the only state for which we produced this type of map was South Carolina. With the
exception of Florida, for which the necessary data were not available, most states in this zone
have marginal breeding numbers with few and relatively small colonies.
FINAL DRAFT Part II. Population Data and Trends, Introduction 11
Trends
To determine trends in breeding numbers, we asked survey recipients to supply data on
all known breeding colonies over 4 time periods: 1970, 1980, 1990, and 1998. Not surprisingly,
many states and provinces did not have data for these specific years, so we asked for data from
censuses conducted around these years. If that was not possible, we asked for whatever data were
available. If complete counts were made for at least two years we generated population trends
(Table 1, Table 4,Table 5,Table 6; see also figures in state and province summaries). To estimate
percent average annual rate of change in number of pairs, we calculated er-1, where
r = ln (recent count) – ln (earlier count) and er = l
years between counts
(Smith 1992). For figures we used all complete data available. For eastern and central North
America (Zones 3, 4, 5), when no complete counts were available, we examined colonies within
a state or province that had > 2 years of data and compared the number of colonies that had
increased at last count with the number that had decreased.
We also provided estimates of size of regional breeding populations for Zones 3 – 5
(Table 4, Table 5, Table 6), based on latest totals of breeding pairs in each state and province that
made up the regional population. We caution that estimates for each state and province may
utilize different survey methodologies, and may not all be obtained in the same year, thus
regional estimates should be considered rough approximations at best. For Zones 1 and 2,
comprehensive data were not available for an estimate of the entire region.
Because data were incomplete for many areas and / or time periods we did not attempt to
calculate a rate of change for the continental or even regional populations. However, Tyson et al.
(1999) reported that the number of DCCOs nesting in the U.S. and Canada increased about 2.6 %
annually from 1990 through 1994. This trend was based on data that ranged from recent
complete counts to conjectures based on old or incomplete information. While the mean percent
annual change in the number of nesting pairs in those states and provinces that had recent
complete counts was 16.2 %, the majority of states and provinces did not have recent complete
counts, and thus Tyson et al. (1999) suggest that these rates of change be used with caution.
Wintering Birds
Locations and Winter Distribution
Survey recipients were asked to provide latitude-longitude coordinates for all known
wintering locations, including feeding and roosting sites where major concentrations (> 1000
birds) occurred. However, the majority of U.S. and Mexican states, and Canadian provinces do
not conduct surveys for winter birds, and could not supply latitude-longitude information for
DCCO wintering areas. Thus it was not possible to create a precise winter distribution map. We
suggest that Hatch and Weseloh (1999) be consulted for winter distribution across the continent.
To provide general information on wintering cormorants, we utilized data obtained through
annual Christmas Bird Counts. A few states (AL, AR, and MS) provided locations for winter
roosts (see below).
Use of Christmas Bird Count Data
The most comprehensive data for wintering DCCOs in the U.S. and Canada were
obtained from annual Christmas Bird Counts (CBCs). Annual CBCs have been the only method
FINAL DRAFT Part II. Population Data and Trends, Introduction 12
to consistently monitor winter birds on a broad scale. While CBCs are not undertaken to produce
precise population estimates, they provide a useful index of population trends and valuable
information on winter occurrence. For some states, CBC data provided the only information
available about wintering cormorants. Therefore, for states with major wintering numbers (> 100
birds overwinter) we obtained CBC data (locations and numbers) collected between 1984 – 1997
for Zone 4 and 1984 – 1998 for Zones 1, 2 and 5. We also obtained data collected between
1984-1998 for states in Zone 3 that border Zones 2,4 and 5. CBC data were accessible for 1984-
1998 at the time of our assessment (Sauer et al. 1996). For each state and province, we included
all CBC sites where > 100 birds were counted at least once during the period under consideration
(see figures in state summaries). These sites are shown in Figure 2.
Wintering Numbers and Trends
In general, data on numbers of over-wintering birds were limited and it was not possible
to determine trends. The information we provide on wintering trends is based mainly on
“qualitative” assessment of apparent changes in numbers counted during CBCs. For some states
(AL, BC, CA, FL, GA, KS, LA, ME, MA, MI, NJ, NY, NC, OK, OR, TN, TX, VA, WA) Sauer
et al. (1996) analyzed CBC data (1959 – 1988) for trends, included in this assessment. CBC data
(1989 – 1999) have not been analyzed, but are included to provide a broad picture of recent
changes in winter numbers. CBC data should be interpreted cautiously, for a number of reasons
(e.g., number of counts is not always consistent within states and provinces; observer skill level
varies).
Limited survey data from Mississippi were included to provide a broader picture of
recent changes in wintering numbers in this state, mainly in the Mississippi Delta region.
Additionally, limited survey data from Alabama, Arkansas and Mississippi data were
incorporated to compare CBC site distribution with known night roosts located by Wildlife
Services surveys (Figure 51). Additionally, comments on perceived trends from biologists and
other individuals or sources providing us with information on winter numbers were incorporated.
At this time, it is not possible to provide an estimate for winter population size because data are
incomplete. However, this may be an important estimate for management actions geared toward
reducing impacts on the breeding and wintering grounds (see Non-breeding Birds and Total
Population Size, below, and PART VII. RESEARCH NEEDS).
Migrant Birds
Migration / Stop-over Sites
Survey recipients were asked to provide latitude-longitude coordinates for all major (>
500 birds) migration stop-over sites. For most states, migrant numbers were very difficult to
obtain because often no information was available, or migrants could not be distinguished from
wintering birds. Some states provided general information about where concentrations of migrant
birds were regularly seen, but because data were so limited we did not map them.
Trends
We provide limited information for a few states and provinces on trends in numbers of
migrants in the specific state/provincial accounts.
FINAL DRAFT Part II. Population Data and Trends, Introduction 13
Non-breeding Birds and Total Population Size
We did not obtain information on non-breeders, which include sub-adult and adult non-breeding
birds. Currently there is very little information about this segment of the population,
and without this information, it is not possible to estimate the continental population size with
any degree of precision. Life-tables have not been established for any of the breeding
populations, age of first breeding can vary, and year-to-year differences in non-breeding by
adults can be significant (Hatch 1995; Hatch and Weseloh 1999). At various times, different
investigators (e.g. Lewis 1929; McLeod and Bondar 1953; Price and Weseloh 1986; Watson et
al. 1991) have estimated that a range of 1.0 – 4.0 non-breeding cormorants per breeding pair
determines the size of the non-breeding segment of North American cormorant populations. In
recent estimates of population size, these estimates have been applied to breeding numbers.
Hatch (1995) estimated the continental breeding population at > 360,000 pairs from data
obtained mainly in the 1980s and early 1990s; applying the range of estimates for non-breeders,
he estimated a continental population size of 1 – 2 million birds. Tyson et al. (1999) estimated a
minimum of 372,410 breeding pairs in the U.S. and Canada, based on data collected between
1975 – 1997 (most data were collected 1994 – 1997). Applying the range of estimates above for
non-breeders, Tyson and colleagues conservatively estimated that > 1 million individual DCCOs
currently occur in the U.S. and Canada. Hatch (1995) noted that totals are imprecise because
large regions have been poorly studied and because some of the largest populations are the least
well known (e.g., Manitoba, Mexico). Additionally, review of the estimates by Lewis (1929),
McLeod and Bondar (1953) and Price and Weseloh (1986) indicated estimates for non-breeders
were not based on detailed scientific study and rigorous data. Before estimates for this segment
can be made with any confidence, careful study and observations at specific colonies (e.g. older,
stable colonies vs. newer, rapidly expanding ones), as well as at locations without colonies where
non-breeders may aggregate, need to be undertaken (D.V. Weseloh, pers. comm.).
In addition to providing important information for total population size, an estimate of
non-breeders is also important for predicting population growth, and effectiveness and impacts
of potential management strategies. For example, at the large Oostvaardersplassen Great
Cormorant (GRCO) colony in The Netherlands, Van Eerden and Van Rijn (1997) estimated that
at least 40 % of birds present did not breed. Based on this estimate, they concluded that any
intervention in colonies aimed at reducing number of breeders is likely to allow non-breeders to
take over the empty territories, or disperse and colonize new territories.
For predicting appropriateness of management strategy, an estimate equally important to
total population size (e.g., regional, continental) is the proportion of birds utilizing various
habitats and resources. For example, Van Eerden and Van Rijn (1997) reported that fish farms
provide food for only about 0.5 – 1 % of the total population of European GRCOs in winter, and
thus managing on the local level was considered more appropriate. We do not have good
estimates for the proportion of the DCCO population utilizing aquaculture farms. For example,
Nisbet (1995) estimated that the number of DCCOs wintering in the Mississippi Valley is
probably equivalent to no more than five percent of the total interior population. However, this
estimate represents the number present at a given point in time rather than the total number of
birds moving through the region, which may be a larger proportion of the total population.
Dolbeer (1991) estimated from banding records that 120,000 birds might move through this
region during winter months. J. Glahn (pers. comm.) suggested that the number currently passing
though has probably more than doubled since Dolbeer’s (1991) estimate. There are two levels of
uncertainty surrounding these estimates. First, we cannot determine the proportion of the
FINAL DRAFT Part II. Population Data and Trends, Introduction 14
population the migrants represent because total population size is unknown. Second, because it is
not clear how many of these birds are overwintering and how many are simply passing through,
it is difficult to determine level of impact this portion of the population has on aquaculture.
Therefore, better estimates on the proportion of the population utilizing fish ponds and the
variation of residence time among birds present in the Mississippi Delta region are important for
decisions about whether to manage cormorants on a population or local level (see PART V.
MANAGEMENT OPTIONS).
FINAL DRAFT Part II. Population Data and Trends, Introduction 15
Figure 1. Distribution of Double-crested Cormorant (Phalacrocorax auritus) breeding
colonies in North America 1970 – 2000.
FINAL DRAFT Part II. Population Data and Trends, Introduction 16
Figure 2. DCCO Christmas Bird Count sites (1984 – 1998) where ³ 100 birds were counted
at least once.
FINAL DRAFT Part II. Population Data and Trends, Zone 1 17
ZONE 1: SUMMARY OF POPULATION DATA FOR ALASKA
Introduction
In Zone 1, the breeding population occurs entirely in Alaska, at Nunivak Island and the
southeast Bering Sea, and from the Aleutian Islands to the southeast coast, including Kodiak
Island. Inland it occurs to Lake Louise; the first nest in the Yukon Territory was confirmed at
Lake Laberge in 1998 (Hatch and Weseloh 1999). The westernmost limit occurs in the eastern
Aleutian Islands at Chuginadak Island (52°51’01” N, 169°49’41” W) (Carter et al. 1995).
Because DCCOs breeding in Alaska represent the entire P.a. cincinatus subspecies (Carter et al.
1995), the largest of the five subspecies (Hatch and Weseloh 1999), the summary of this
population zone is fully treated in the Alaska state summary, below.
Alaska
Summary of Population Data and trends
(Surveys completed by Don Dragoo, Alaska Maritime NWR, USFWS, Homer; and Shawn
Stephenson, Migratory Bird Management, USFWS, Anchorage.)
Breeding.
While we did not determine when DCCOs were first documented as a nesting species in
Alaska, it is clear that the species has been a long time breeder in the state (see Historical
Information, below). Between 1970 and 2000, the species was confirmed breeding at 126
colonies. Most breeding occurs along the southern coast and on the Aleutian Islands (Figure 4).
Because Alaska has not conducted statewide censuses of all colonies in the same year, the
number of breeding birds is not known. Additionally, most colonies have only been censused
once or not since the 1970s (Carter et al. 1992); therefore population trends are not available.
However, of 126 known colonies, 106 were counted at least once between 1970 and 2000; the
vast majority (93 %) were small, with < 100 pairs on average. Carter et al. (1995) estimated a
total of 2,811 pairs bred at 90 coastal colonies between 1970 and 1992, at which time comprised
12 % of the Pacific Coast population, defined by Carter et al. (1995) to encompass P.a.
cincinatus in Alaska, and P.a. albociliatus, breeding from southern British Columbia to Sinaloa,
Mexico. In the interior, colonies have not been adequately surveyed.
Numbers are thought to have declined since historical times, especially after introduction
of predators (see Historical Information, below). At present, oil spills threaten cormorants in
several areas, and human disturbance at breeding colonies remains a problem (Carter et al.
1995). On coastal islands, introduced predators [e.g., red and arctic fox, Norway rat, ground
squirrel (Spermophilus undulates), rabbit are being removed], which may enable former breeding
sites to be re-colonized by cormorants (Bailey 1993; Carter et al. 1995).
Winter.
In winter DCCOs occur, but no information was provided on wintering locations and
winter censuses are not conducted. Gabrielson and Lincoln (1959) reported the species was
resident in its breeding range, except possibly in the more northern areas, and that it was a fairly
common winter bird in southeastern Alaska. During CBCs conducted between 1984 – 1998, only
3 sites had > 100 DCCOs during at least one count, and fairly small numbers were observed
FINAL DRAFT Part II. Population Data and Trends, Zone 1 18
overall (Figure 3). The CBC data have not been analyzed by the Patuxent Wildlife Research
Center for trends during this period, and no trends are readily apparent from Figure 3.
Migration.
No information was available on migrants or migration in Alaska.
Historical Information
The DCCO is a long time resident and breeding species in Alaska. Remains of DCCOs
found in middens on Amchitka Island date back 2,650 years (Siegel-Causey et al. 1991). Midden
remains indicate DCCOs were formerly more abundant in the central Aleutian Islands, and
climate changes are thought to have probably reduced numbers in Alaska over time (Carter et al.
1995). While it has been suggested that the introduction of arctic foxes onto nesting islands
beginning in 1750 (Bailey 1993) probably greatly reduced breeding numbers (Carter et al. 1995),
it is not entirely clear how severely these introductions impacted DCCOs. In the late 1800s,
DCCOs were reported as abundant residents and breeders in the Near Islands in the western
Aleutians (Turner 1885), where foxes at this time were already present; thus whatever damage
foxes had done would already have been evident (V. Byrd, pers. comm.).
Land Ownership
No information provided.
Productivity
In 1998, productivity was monitored at Aiktak Island, and 1.8 chicks / nest fledged (Byrd
et al. 1999). On Duck Island, productivity was monitored in both 1998 and 1999, and was 0.0
and 0.13, respectively (Byrd et al. 1999; D. Dragoo, pers. comm.).
Figure 3. December estimates of DCCOs in Alaska based on CBCs, 1984 – 1998.
0
50
100
150
200
250
300
84 85 86 87 88 89 90 91 92 93 94 95 96 97 98
Year
Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 1 19
Figure 4. Distribution and size of active colonies in Alaska at time of last surveys (1970 –
1999).
FINAL DRAFT Part II. Population Data and Trends, Zone 2 21
ZONE 2: SUMMARY OF POPULATION DATA FOR THE PACIFIC COAST
Introduction
Breeding Information
In this zone, the breeding population occurs mostly along the coast from southern British
Columbia south to at least Bird Island, Sinaloa, Mexico; some birds may nest farther south
(Carter et al. 1995; Hatch and Weseloh 1999). Birds in this zone comprise the P.a. albociliatus
subspecies (Baird et al. 1884; Palmer 1962). Though most nesting occurs along the coast,
significant colonies also occur or have occurred inland in Washington, Oregon, California and
Mexico. Additionally, most birds breeding in Nevada, Arizona and New Mexico are presumed to
be members of this subspecies. However, Johnsgard (1993) suggested that birds breeding in
western Nevada at Pyramid Lake would seem to be more geographically affiliated with auritus,
but we have chosen to group the Nevada birds in population Zone 2 based on geographic
proximity to California and Arizona birds. In New Mexico, most of the breeding birds are
probably P.a. albociliatus, but a small number likely represent auritus (S. Williams, pers.
comm.). At this time we decided to include New Mexico birds in the Pacific Coast population
because auritus numbers appear to be marginal and data are still being analyzed (see New
Mexico profile for further detail). Birds breeding in Idaho and Utah have characters intermediate
between P.a. albociliatus and auritus, but these birds have been grouped in Population Zone 3
(see summary for Zone 3: Summary of Population Data for Canadian and U.S. Interior
Population for further information). Table 1 shows the status (Breeding, Wintering, Migrant) of
the Double-crested Cormorant throughout Zone 2.
No survey for this assessment was completed by several states in this region because
breeding data for much of the Pacific Coast have been compiled into one database by the
USFWS, Office of Migratory Birds and Habitat Programs-Pacific Region. We used this
database to acquire information on cormorant distribution and abundance.
For many states and provinces in this zone and Alaska, we cite percent of the Pacific
Coast population that breeding pairs in a particular area comprised, based on data from Carter et
al. (1995). They define the Pacific Coast population to encompass P.a. cincinatus in Alaska, and
P.a. albociliatus, breeding from southern British Columbia to Sinaloa, Mexico. Therefore, when
we give estimates of percent of the Pacific Coast population that pairs from a particular area
comprised, we refer to the Pacific Coast population as defined by Carter et al. (1995). Though
their definition of the Pacific Coast breeding population differs from ours, we cite Carter et al.’s
(1995) percent estimates to provide additional information on the number of birds occurring
within this region.
Along the coast, from British Columbia to Mexico, Carter et al. (1995) estimated that
approximately 21,849 pairs nested between 1968-1992. More recent estimates for the entire
region are not available, but significant changes occurred during the 1990s. Large increases were
documented along the coast in Oregon at the mouth of the Columbia River, and inland in
California at the Salton Sea; increases in these two areas account for most of the growth in the
region. Sharp declines were observed along coastal British Columbia and Washington colonies.
Because data are not available for birds breeding in significant portions of this region
(e.g., Mexico, some interior areas) and data collection has not been coordinated at a regional
scale, it is not possible to summarize trends for the population as a whole. Additionally, trends in
this region are strongly affected by apparent movements of birds during El Nino oceanographic
FINAL DRAFT Part II. Population Data and Trends, Zone 2 22
conditions, habitat loss at interior colonies, and use of artificial habitat in some areas (Carter et
al. 1995). With these habitat changes, distributional changes appear to be occurring; large
numbers of immigrants may account for much of the growth seen in particular areas (see
California and Mexico profiles). How these distributional changes affect overall population
growth for Zone 2 is not clear.
Winter and Migrant Birds
Though portions of the Pacific Coast and the interior provide wintering habitat for
DCCOs breeding in this region, no state- or province-wide counts of wintering DCCOs have
been conducted by state or provincial agencies monitoring cormorant numbers. Therefore, with
the exception of British Columbia, which reported that all breeding DCCOs are residential, no
precise estimates are available for winter numbers in this region. Additionally, while many of the
birds that breed in this zone are year round residents, some migration does occur, and it is
difficult to distinguish migrants from residents. Overall estimates of migrants in the region have
not been obtained.
Analysis of recent (1989 – 1998) CBCs conducted in this region has not been undertaken,
and trends are not readily apparent from data collected during counts conducted between 1984 –
1998 (Sauer et al. 1996) (Figure 5). However, CBCs indicate both coastal and interior areas of
California provide major winter habitat; numbers from California counts are much larger than
those reported in any of the other states (but see California profile for limitations of this data).
CBC data also suggest that coastal areas in British Columbia, Washington and Oregon provide
winter habitat for significant numbers of birds. In the interior, significant numbers are reported in
Arizona on larger reservoirs along the Colorado River, especially near Yuma, and in New
Mexico in the Elephant / Butte Caballo and Carlsbad areas (Figure 2).
Several areas in Mexico also provide significant winter habitat. The DCCO is commonly
found along both coasts of Baja Sur, and along the Gulf Coast of Tamaulipas and Campeche. It
is also a fairly common to common resident from the northwest Yucatan to northern Belize
(Howell and Webb 1995). However, no information is available on winter numbers in Mexico.
Historical Information
Carter et al. (1995) rev